List of The Big Bang Theory characters
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The series initially centers on five characters: roommates Leonard Hofstadter and Sheldon Cooper, two Caltech physicists; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's friends and co-workers aerospace engineer Howard Wolowitz, and astrophysicist Raj Koothrappali.
Over time, several supporting characters have been introduced and promoted to starring roles, including Bernadette Rostenkowski-Wolowitz, Amy Farrah Fowler, Stuart Bloom, Emily Sweeney, and Leslie Winkle. The series also features numerous supporting characters, each of whom plays a prominent role in a small group of episodes. Included among them are parents of the main characters, their dates and their coworkers. Celebrities such as Stephen Hawking occasionally appear in cameo roles as themselves.
- 1 Character appearances
- 2 Original main characters
- 3 Additional main characters
- 4 Recurring characters
- 4.1 Alex Jensen
- 4.2 Alfred Hofstadter
- 4.3 Arthur Jeffries
- 4.4 Barry Kripke
- 4.5 Bert
- 4.6 Beverly Hofstadter
- 4.7 Claire
- 4.8 Dave Gibbs
- 4.9 Dr. Eric Gablehauser
- 4.10 Dr. V.M. Koothrappali
- 4.11 Mrs. Koothrappali
- 4.12 Janine Davis
- 4.13 Kurt
- 4.14 Lucy
- 4.15 Mary Cooper
- 4.16 Mike Rostenkowski
- 4.17 President Siebert
- 4.18 Priya Koothrappali
- 4.19 Stephanie Barnett
- 4.20 Wil Wheaton
- 4.21 Mrs. Wolowitz
- 4.22 Zack Johnson
- 5 Minor characters
- 6 Guest stars
- 7 Notes
- 8 References
- 9 External links
Original main characters
Leonard Leakey Hofstadter (Johnny Galecki) is an experimental physicist with an IQ of 173. Originally from New Jersey, he received his PhD at age 24, spending at least some of his time at Princeton University. He shares an apartment in Pasadena with colleague and friend Dr. Sheldon Cooper. Unlike his equally geeky friends, Leonard is interested and quite adept in building interpersonal and social relationships with other people. Compared to his friends, he is relatively successful with women.
The writers have toyed with a romance between him and next-door neighbor Penny since the series premiere, with their unresolved sexual tension being a major force for drama. Leonard dated Penny for most of season three, although they had dated briefly at the end of season one. Aside from Penny, Leonard has had romantic relationships with co-worker Leslie Winkle, physician Stephanie Barnett, an unnamed French literature PhD, and North Korean spy Joyce Kim. Following his break-up with Penny, Leonard began a relationship with Raj's sister Priya in season four, and it is implied they had a brief sexual encounter before the start of the series. The two attempted a long-distance relationship after Priya moved back to India, but Leonard eventually broke up with her in season five after she admitted to cheating on him with her former boyfriend. In season five's "The Recombination Hypothesis", Leonard and Penny start dating anew, and although their relationship is initially rocky and Penny has doubts, in season six, she works through her doubts and has a stronger relationship with Leonard.
In season six's "The 43 Peculiarity", she tells him she loves him for the first time, and this now makes them more comfortable in their relationship. Over the next two years, they talk about marriage and each have unsuccessful proposals until the penultimate episode of season seven, when they become engaged, and finally get married in the ninth season premiere. Leonard's family includes other accomplished scientists: his mother, Dr. Beverly Hofstadter, is a neuroscientist and world-renowned psychiatrist, his sister is a medical researcher, and his father, Dr. Alfred Hofstadter, is an anthropologist. Also, his brother Michael is a Harvard law professor. He is known to be lactose intolerant, and is also known for playing the cello.
Sheldon Lee Cooper (Jim Parsons) is a theoretical physicist, possessing a BS, MS, PhD, a DS, and an IQ of 187. Originally from East Texas, he was a child prodigy, starting college at the age of 11, and receiving his PhD at age 16. He is usually cold, condescending, selfish, and immature, but at the same time it is commonly not intentionally hurtful and is in a well-meaning way; He just doesn't know any better. He does have a softer side, as he does care about his friends, and loves his mom, grandma, and girlfriend Amy, though he often insults them with either about their intellect levels or different opinions. He shares an apartment in Pasadena with friend and fellow physicist Dr. Leonard Hofstadter.
Sheldon has a deep love and interest in science fiction and comic books, to the point of being obsessed about them, and shows particular affinity for Spock, a fictional character from the Star Trek franchise. He wears clothing emblazoned with logos for these interests as well as science-related designs. Sheldon refrains from any form of physical contact if possible and exhibits a strict adherence to routines to the point of exhibiting signs of obsessive–compulsive personality disorder. Any disruption to routines distresses him enormously. A running gag has him always knocking on doors in a pattern of three knocks and one call repeated three times, which he has the urge to complete no matter what.
Sheldon has major difficulties in social settings; he shows great difficulty in identifying sarcasm and irony, and a recurring topic is his efforts to acquire those skills. His commitment to his friends is often called into question, as he is known for constantly and consistently being condescending and rude, many times even without being intentionally hurtful. He also has difficulties lying or keeping a secret, giving himself away with wild facial tics and implausible statements. He is, however, fond of occasionally telling a joke or playing a prank, which he typically punctuates with his trademark exclamation "Bazinga!". He has forced Leonard into signing an absurdly exhaustive "Roommate Agreement", and later enters a similar "Relationship Agreement" with Amy Farrah Fowler. Sheldon has a superiority complex and makes no effort to hide his contempt for other people's lower intellects, often making highly inappropriate comments. A common joke is his inability and unwillingness to drive, relying on those around him to help him get anywhere.
Unlike Leonard and Raj, who come from intellectual families, Sheldon stands out from his own relatives. His father, George, whose death antedates the series, is described as a stereotypical Texan who liked football, skeet shooting, and heavy drinking; his mother, Mary, is a devout Evangelical Christian, who does not seem to understand her son's fields of study, although she does display common sense and intelligence; his fraternal twin sister, Missy, and as-yet unseen brother, George Jr., were both described by Mary as being "dumb as soup". His sister later has a baby boy, making Sheldon an uncle.
He meets Amy Farrah Fowler in the third season finale and starts a highly unusual and very slowly evolving relationship with her. Prior to this, he had exhibited some affinities with Leonard's equally unemotional mother. In the eighth season finale, Amy takes a break from Sheldon, although the two resume their relationship halfway through the ninth season; as a sign of his love towards her, Sheldon decides to have sexual intercourse with her for her birthday in "The Opening Night Excitation". In the tenth season, Sheldon and Amy move in together for a five-week experiment to test out their compatibility.
Penny Hofstadter (maiden name has not been revealed) (Kaley Cuoco), is Leonard and Sheldon's neighbor across the hallway. Originally from a small town outside Omaha, Nebraska, she was a waitress and occasional bartender at the local Cheesecake Factory until season seven, and is an aspiring actress. According to her friend and neighbor Sheldon, she is a "good-natured simpleton". She is kind-hearted, outgoing, and does display the most common sense out all the original five main characters. She also appears to be the most street-smart. Her lack of educational qualifications (she dropped out of community college) are a constant source of disparaging comments from Sheldon.
She has dated several men during the course of the series, including Kurt, Stuart, and Zack, but her recurring romantic interest is Leonard. Leonard pursued Penny during season one; they dated briefly at the start of season two, which led to some awkwardness; they began dating again and were a couple for most of season three, but she broke up with him after he told her he loved her and she was not ready to reciprocate. Penny and Leonard begin to date again "slowly" during the fifth season. After several unsuccessful proposals from both sides, Penny realizes that her happiness is derived from her relationship with Leonard and they become engaged and later get married. She has a tense relationship with Sheldon, whose nerdy and obstinate personality often exasperates her, but they are ultimately good friends, helping each other out in various situations. By season four, Penny also begins to socialize more with Bernadette and Amy, who frequently hang out in Penny's apartment, go out together, and comfort each other. Bernadette and Amy are both shown to be simultaneously envious of and threatened by Penny's more outgoing personality. Not much is known about Penny's family, but it is mentioned in the series that her father, Wyatt (portrayed by Keith Carradine,) raised her like a boy, her mother smoked marijuana while she was pregnant with her, her sister shot her husband while they were intoxicated, and her brother is a meth dealer.
At the start of season eight, Penny pursues a career in pharmaceuticals and interviews for a job at Bernadette's workplace. She gets the job, and gradually becomes more independent, including earning her own high salary; however, she reveals in season nine that she's not happy with her job.
During her marriage to Ryan Sweeting, Cuoco was credited as Kaley Cuoco-Sweeting, starting with the season-seven episode "The Convention Conundrum", but she returned to being credited as Kaley Cuoco after they separated in 2015.
Howard Joel Wolowitz, MD (Simon Helberg) is an aerospace engineer at Caltech's Department of Applied Physics, who often hangs out at Leonard and Sheldon's apartment. Unlike Sheldon, Leonard, and Raj, Howard has no doctorate, and often gets disparaged as a result, especially by Sheldon. He defends this by pointing out that he has a master's degree in Engineering from MIT and that the equipment he designs is launched into space, unlike the theoretical work of his friends. In the fifth season finale, Howard goes to the International Space Station on Expedition 31.
Howard lives in Altadena with his domineering, belittling, and unseen mother, who treats him like a child. While he sometimes expresses irritation at this treatment, for the most part, he appears to prefer it. Howard dotes on his mother and participates in many of her daily routines. A recurring gimmick in the series is Howard and his mother communicating with each other from different rooms by yelling, a habit which Bernadette also adopts in later episodes. His father left them when he was 11, and Howard gets visibly distressed when the subject comes up. Howard fancies himself a ladies' man and attempts pick-up lines whenever a woman is present, which often come off as inappropriate and disgusting, although he has dropped this habit since he started going out with Bernadette. He is Jewish, but he is not very serious about his faith, and does not keep kosher. However, he does observe the Sabbath and the Jewish holidays.
In the pilot episode, he speaks English, French, Mandarin, Russian, Arabic, and Persian, along with fictional languages like Klingon and Sindarin. He is asthmatic, allergic to peanuts, almonds, and walnuts, prone to canker sores and pink eye, succumbs to seasickness, has an incredibly high genetic risk of heart illness, and has transient idiopathic arrhythmia. Howard dated Bernadette briefly in season three. After some difficulties, they resumed their relationship in season four, which culminated in their engagement. In the fifth season finale, Howard and Bernadette are married on the roof of the apartment building shortly before he has to leave for Kazakhstan, where he is to be sent to the International Space Station. In season six, he finally leaves his mother's house and moves into Bernadette's apartment. In the episode "The Comic Book Store Regeneration", Howard's mother passes away in her sleep while in Florida, prompting him to move back into his home with Bernadette. Stuart, the owner of the guys' favorite comic book store, joins them, much to their annoyance.
Upon finding out that Bernadette is pregnant in season nine, Howard freaks out about how to handle the baby situation, particularly earning enough money. With the help of his friends, he designs a new form of guidance system that attracts the attention of the Air Force.
Rajesh Ramayan Koothrappali, PhD (Kunal Nayyar) is Howard Wolowitz's best friend and another genius of the group; his name is usually shortened to "Raj". He is originally from New Delhi, India, and he works in the physics department at Caltech, where his area of expertise is astroparticle physics. He lives in an apartment in Pasadena. As with his friends, he is mutually involved with and obsessed with science fiction and comic books in general. He is also a fan of Harry Potter.
Raj dislikes India, Indian food and Indian music, but appreciates the Indian lullabies his mother sang to him and the catchiness of Hindi phrases. Raj is a Hindu and believes in karma (reincarnation), but eats beef. He is very shy around women outside of his family, and during the first six seasons of the show, found himself unable to speak to women while in the presence of them unless he drank alcoholic beverages, or he believed he had done so. When Penny, Amy, and Bernadette were around, Raj usually whispered what he wanted to say to Howard or Leonard, who then repeated or responded to what Raj said out loud. Despite this, Raj has sometimes ended up in bed with women, leaving the other guys perplexed.
Unlike his friends, Raj has many quite feminine interests, such as reading Archie comics and Twilight and watching chick flicks such as Bridget Jones's Diary. A recurring joke in the series has people, including his parents and Leonard's mother, speculating that Raj might be gay due to his feminine interests and his close friendship with Howard, with whom he has arguments similar to those of a married couple. However, Raj has always stated that he is straight, but metrosexual. He has had an infatuation with Penny, and secretly wrote love poems about Bernadette. Raj comes from a very wealthy family in India, and often communicates with his parents, Dr. and Mrs. Koothrappali, via webcam. He has a younger sister, Priya.
In season six, Raj started seeing a shy and sweet woman named Lucy, who had many things in common with Raj. However, she feels too pressured when he asks her to meet his friends and breaks up with him. Afterwards, Raj finally speaks to Penny without having consumed alcohol. As of season seven, he no longer requires alcohol, drugs, or medication to speak in front of women. In season seven, Raj starts dating Emily Sweeney, a dermatologist, and their relationship becomes exclusive at the end of the seventh season. In the ninth season, he meets a bartender/scriptwriter and dates her along with Emily. However, in season ten, he mentions he is currently single.
Additional main characters
Leslie Winkle, PhD (Sara Gilbert) is an experimental physicist who works in the same lab as Leonard. In appearance, she is essentially Leonard's female counterpart, equipped with the black-framed glasses and sweat jackets. She also happens to have a lazy eye. She is one of Sheldon's arch-enemies, due to their conflicting scientific theories. Though they both consider each other to be intellectually inferior, Leslie is much wittier than Sheldon, regularly calling him "dumbass" and besting him in their exchanges. Leslie has had casual sexual relationships with both Leonard and Howard, considering it a completely physical reaction to her body's cycles.
Sara Gilbert made her first appearance in the third episode of season one and her final appearance as a regular in the third season finale; Entertainment Weekly reported in January 2009 that she had been downgraded from starring status to recurring because the producers of the series were unable to come up with new storylines for the character. She returned for a guest role in the 200th episode, "The Celebration Experimentation", in season 9, with her character talking briefly about how she and Sheldon have changed over the years and affectionately calls him "dumbass" again at Sheldon's insistence (due to his dislike for changes).
Bernadette Maryann Rostenkowski-Wolowitz, PhD (Melissa Rauch) is originally a waitress and co-worker of Penny's at The Cheesecake Factory, which pays the way for her graduate studies in microbiology. She is a smart, short-tempered, ruthless, and often competitive young woman who also displays a sweet, nice, and friendly personality. Despite her high-pitched voice, she has a darker side. A recurring joke in the series is in reference to Bernadette and her lab team handling dangerous or infectious specimens, leading to accidental byproducts or exposure to diseases, in some cases resulting in Bernadette ending up in quarantine.
Bernadette is of Polish origin and Catholic upbringing, and is frequently seen wearing a cross necklace. She is generally a friendly and good-natured individual, quickly making friends with Penny and Amy, as well as the rest of the gang. However, despite her short stature, she is regarded as being somewhat intimidating, largely because of her sharp tongue, aggressive demeanor, and occasional signs of mania. In season eight's "The Locomotion Interruption", Penny is interviewing for a pharmaceutical sales representative position with Bernadette's boss, and Penny and the boss both agree that Bernadette is kind of a bully.
Bernadette has demonstrated that she has some influence over both her father and her husband, including directly supervising Howard's expenditures by giving him allowance money. On one occasion, she was also able to influence Sheldon into a decision of which he originally disapproved.
Penny first introduced Bernadette to Howard on Leonard's half-willing request. During their first date, Bernadette and Howard do not find any common field of interest until they discover that they both have domineering mothers. After their third date, Bernadette declares that three dates is the threshold for deciding whether they want a long-term relationship. Blindsided, Howard hesitates initially, causing a temporary rift between them, but then impulsively proposes marriage to show her that he does care about her. Frustrated, she declines, but when he sings a heartfelt song for her at the Cheesecake Factory, she declares that it is the most romantic thing anyone has ever done for her and their relationship continues. A lengthy offscreen breakup occurs when she catches Howard engaging in cybersex, but Howard apologizes and she consents to restarting their relationship.
In the fourth season, she obtains her doctorate and is subsequently hired by a pharmaceutical company that offers her a high salary. While preparing for their wedding, Bernadette is shocked and repulsed when she learns about Howard's past sexual escapades, although once again, an apology and reconciliation occurs in the same episode. They marry the day before Howard leaves for his upcoming mission aboard the International Space Station.
During seasons six and seven, the newlyweds deal with Howard's difficulty in readjusting to life on Earth, the struggle to keep a healthy relationship with Howard's mother, and arguments over money. In the ninth-season episode, "The Valentino Submergence", Bernadette reveals that she is pregnant, informing Howard in the next episode. She comically reveals that the baby was conceived in Sheldon's bed, much to Sheldon's chagrin.
Amy Farrah Fowler, PhD (Mayim Bialik) is a neuroscientist and Sheldon's love interest in the series. She has a PhD in neurobiology (Bialik herself has a PhD in neuroscience), with a research focus on addiction in primates and invertebrates, occasionally mentioning such experiments as getting a capuchin monkey addicted to cigarettes or starfish addicted to cocaine.
Raj and Howard found Amy through an online dating site after secretly setting up an account using Sheldon's name and information. The site matches her to Sheldon, and the two share many similar traits. Once she and Sheldon meet, she becomes, as Sheldon puts it, a girl who is his friend, but not his "girlfriend". The two initially communicate by computer, but at the encouragement of the group, she starts socializing with the gang in person.
Early on, Amy is essentially a female counterpart to Sheldon. She is coldly rational, displays little emotion, and is awkward in social situations ranging from endearing to embarrassing. In season five, after Amy goes out on a date with comic-book store owner Stuart, Sheldon decides to solidify their relationship as boyfriend/girlfriend with a 31-page "Relationship Agreement". Later in the season, Amy begins a campaign to increase Sheldon's feelings for her by becoming more involved in his interests, including video games and Star Trek, and treating him as his mother did. Sheldon says during Howard and Bernadette's wedding ceremony that he hopes the two of them are as happy together as he is alone, hurting Amy's feelings, but he spontaneously takes her hand for comfort later as Howard is launched into space.
By the beginning of season six, they are regularly holding hands at Amy's insistence, but Sheldon repeatedly resists all of her attempts to engage in further physical intimacy. By mid-season, Sheldon gets some intimate views of Amy when he cares for her while she has the flu, pushing him further toward a traditional relationship with her. She also provides Sheldon with consoling hugs, and was slapped on the rear by an intoxicated Sheldon telling everyone that she is a great gal.
Amy tries to move into Sheldon's apartment when Leonard temporarily vacates it, but has to give that up. Sheldon, after being prodded by Barry Kripke about his girlfriend, lies that he is having sex with Amy, though he admits to Penny that it might really happen between Amy and him, and later admits it to Amy, while also engaging in Dungeons and Dragons intercourse game play. Nevertheless, her frustration is frequently played for laughs from that point onward. In the season-seven episode "The Locomotive Manipulation", Sheldon sarcastically says he will give Amy the romance she wants, then passionately kisses her on the lips for the first time and enjoys it. In "The Prom Equivalency", Sheldon and Amy admit to being in love with each other.
While early on Amy was ambivalent and often condescending towards Penny and Bernadette, she later spends more time with them, even at the expense of her time with Sheldon. As her appearances have progressed, she has shed her strident, aloof personality for a stereotypically feminine and social one, although she retains some social awkwardness. Her feelings for Sheldon have also grown considerably over time, and she considers him to be ideal in almost every way, though she sometimes becomes as annoyed and frustrated by Sheldon's quirks as his other friends do.
Amy also intensely wants her relationship with Sheldon to progress to include sexual intimacy, a drastic departure from her early days as a member of the circle of friends. She has come to consider Penny a very close friend, sometimes referring to her as her "bestie", occasionally admitting her entire social life revolves around Penny. Penny is fond of Amy, but to her dismay, Amy often fails to grasp the concept of "girl talk", by talking about feminine hygiene and anatomy when in her presence and at times being brutally honest. Amy also often demonstrates an infatuation towards Penny, occasionally bordering on physical attraction, and she has demonstrated a somewhat condescending attitude towards Bernadette, though still considering her a close friend.
Amy can play the harp. In "The Scavenger Vortex", she mentioned that she attended Harvard University. In the final episode of season eight, "The Commitment Determination", Amy decides to take a break from her relationship with Sheldon due to the lack of physical affection. Sheldon had intended to propose to Amy, but she took a break from the relationship before he could ask her. Amy ultimately breaks up with Sheldon after a serious falling out, but they slowly rekindle their relationship and eventually have sex as a birthday present from Sheldon.
When Sheldon's grandmother visits in season 9, she confronted Amy for hurting Sheldon and unintentionally revealed the secret engagement ring in his possession. Although Amy was initially shocked, she became really happy to know that Sheldon loves her enough to have considered taking that step forward in their relationship. Amy later prepares a surprise birthday party for Sheldon, which he has not previously celebrated with the group.
After a plumbing accident makes her apartment uninhabitable in season 10, Sheldon and Amy embark on a five-week experiment living together to test out their compatibility.
Stuart David Bloom (Kevin Sussman) runs the Comic Center of Pasadena, the comic-book store that the guys visit. Stuart is characterized by his low self-esteem and loneliness, which often result in pathetic attempts to engage with women and win favor with the main cast. This is despite owning his own (albeit troubled) business, and being a talented portraiture artist who attended the Rhode Island School of Design.
During Stuart's first appearances in season 2, he showed enough social skills to ask Penny on a date on two different occasions. Neither of the dates ended successfully, however. In season five, he also briefly went on a date with Amy, although Sheldon interrupted the date to solidify his relationship with her.
Stuart was the partner of Wil Wheaton in a trading-card game tournament in which they defeated the team of Raj and Sheldon. As Stuart runs a comic-book store, he has an almost encyclopedic knowledge of comic books and superheroes. In the season-four episode "The Toast Derivation", he mentions that he is in financial trouble and that the store is now also his home, and over the years, he has questioned the profitability or success running the store.
During "The Decoupling Fluctuation", Stuart becomes close friends with Raj and starts hanging out with the gang as a replacement for Howard while Howard is in space. When Howard returns, Sheldon wants to dismiss Stuart from the group, but Raj objects. Leonard tie-breaks on the basis that they are receiving a 20% discount in Stuart's store, and Sheldon relents after Stuart offers to go as high as 30%, thereby becoming a bigger and more prominent part of the core group.
After his comic-book store burns down, Howard offers him a place to stay and a job as his mother's nurse. Stuart likes the job and develops a surprising closeness with Howard's mother, even staying there long after she had recovered, much to Howard's annoyance. After the death of Howard's mother, Howard and Bernadette move back into the house, with Stuart still staying. In the eighth season finale, the two attempt to ask Stuart to move out, but are unable to bring themselves to do it. He eventually made the decision himself to move out of the house in the middle of season 9.
Kevin Sussman was a recurring guest actor from seasons two to five, and was promoted to starring role for the sixth season onwards.
Emily Sweeney, MD (Laura Spencer) is a dermatologist at Huntington Hospital, whom Raj finds on an online dating site. Emily went to Harvard, and is shown to have a slightly alarming personality; she delights in the macabre.
In season seven, after finding Emily's online dating profile, Raj has Amy contact her. Amy and Emily exchange emails and find they have a lot in common. Emily does not like Raj's inability to correspond with her directly, but when he does show up to meet her, he acts weirdly and she leaves. Raj later bumps into Emily and apologizes for their previous encounter. She gives him another chance and agrees to go out to dinner with him. In "The Relationship Diremption", Raj and Emily have dinner with Howard and Bernadette. Upon meeting Emily, Howard recognizes her from a blind date they had sometime before he met Bernadette. The blind date ended badly, causing him embarrassment at the dinner, but not any issues.
In "The Gorilla Dissolution", Raj and Sheldon bump into Emily at the movies with another guy. Later, she visits him to talk and reveals that the guy was her tattoo artist who had been asking her out for weeks and she did so just to end the obligation. Emily then spends the night and in the next episode, Raj reveals to the gang that Emily and he are exclusive.
In season eight, Raj introduces Emily to Leonard and Penny; however, Penny senses that Emily does not like her. Emily reveals that she is uncomfortable with the fact that Raj and Penny previously hooked up. Emily and Penny later sit down to settle their differences, with mixed results. Emily continues to socialize with the gang in season 8, and in the finale, she suggests to Raj that they have sex in a graveyard. Raj wonders whether he should break up with her due to her alarming personality. The next season, Raj somewhat rashly breaks up with Emily after meeting another girl, Claire, and fantasizing about starting a family with her, though Claire makes amends with her boyfriend and berates Raj for his actions. He tries to reconcile with Emily, but she shuts the door in his face. Later in the season, Emily and Raj reconcile after a tearful phone call. However, in season 10, Raj mentions that he is currently single.
Laura Spencer was a recurring guest actress on the series in seasons seven and eight. During production of season 9, she was promoted to a "fractional" starring role; the upgraded status ensured her availability on an as-needed basis, without requiring her in every episode.
The following list is sorted in alphabetic order of first names.
Alex Jensen (Margo Harshman) is an attractive Caltech graduate student who appears in season six. She is hired by Sheldon to review his kindergarten and elementary-school notebooks for any possible Nobel Prize-winning research and attend to the tasks that Sheldon perceives a waste of his own valuable time. Amy is at first jealous until she sees Alex instead flirting with an unresponsive Leonard.
After repeated flirts by Alex, Leonard finally realizes that Alex is making advances toward him. While proclaiming his loyalty to Penny, Leonard is flattered. Sheldon interferes (with the perceived intention of mitigating a situation that needed none) by lecturing Alex, but only succeeds in insulting her. After Alex files a sexual harassment complaint, Sheldon is forced to apologize and is ordered take a university-mandated online sexual-harassment course. Because Sheldon considers his own time as too valuable to waste on it, Sheldon asks Alex to take the course, instead. For Valentine's Day, Sheldon sends Alex out to buy a present for Amy, which he likes so much, he keeps it himself. 
Alfred Hofstadter (Judd Hirsch) is Leonard's father, an anthropologist who once worked with famed archaeologist Louis Leakey. He was miserable in his marriage to Leonard's mother Beverly, who was always a pain in the ass and was unmotherly towards their children. Alfred claimed she was cold and emasculating, and had not shown him any physical affection for years. On another bit, when Leonard was six, he walked in on his parents naked, and she was swatting his rear end with Leonard's brand-new ping-pong paddle, and as a result in season three, the two were revealed to be getting divorced after he had had an affair with a waitress.
In his first appearance in "The Convergence-Convergence" during his visit for Leonard and Penny's rewedding, he meets Mary Cooper and the two get along great as they have each other's nice, gentle, and caring personalities, bonding over their mutual dislike of Beverly, and their admiration of each other's beliefs. The two of them end up fighting with Beverly at the restaurant during the pre-ceremony dinner and the two leave together and after finding out they are staying at the same hotel, they decide to share a cab and it is implied that the two hook up.
In "The Conjugal Conjecture", Alfred is revealed to have not slept with Mary, but they still plan on seeing each other in the future. In the ceremony, he says that though Beverly and he do not bring the best out in each other, something wonderful did come out of their relationship: Leonard. Beverly agrees. On the way to the airport, they appeared to be civil for about a minute until they started bickering over Leonard's driving.
Arthur Jeffries, or Professor Proton (Bob Newhart) is the star of a science show that Sheldon and Leonard watched as children. After the show was cancelled, Jeffries was not taken seriously as a scientist and resorted to doing children's parties as his persona. In "The Proton Resurgence", Sheldon hires him to do a private party for Leonard and himself (and Penny). Later, Jeffries ends up having a heart attack as a result of climbing the apartment stairs with his equipment. He asks Sheldon to take his place at a party for a Korean family, which Sheldon agrees to do as "Professor Proton, Jr".
While Sheldon idolizes him, Jeffries is constantly irritated by Sheldon's personality, and takes a shine to Penny. In "The Proton Displacement", Leonard, Sheldon, and Amy bump into Jeffries at a drugstore, and Sheldon becomes upset when Jeffries asks Leonard for help on a paper instead of him, and when he finds out that Jeffries thinks he is annoying, Sheldon works with Bill Nye the Science Guy, whom Jeffries accuses of stealing material from his TV show. After Nye abandons Sheldon and files for a restraining order against him, Jeffries arrives at Sheldon and Leonard's apartment and asks Sheldon for his advice on his paper (although Sheldon already hacked his email account and read it for himself).
In "The Proton Transmogrification" on Star Wars Day, Leonard tells Sheldon that Jeffries has died. Sheldon immediately pretends like he does not care and refuses to go to the funeral. Jeffries appears to Sheldon in a dream, where Sheldon compares him to Obi-Wan Kenobi, dressing him in Jedi robes and making him appear as a Force ghost. When Sheldon asks for his wisdom, Jeffries tells Sheldon to appreciate everything in life, including his friends, and never take it for granted.
Jeffries returns in "The Opening Night Excitation" when his ghost convinces Sheldon to spend time with Amy on her birthday instead of going to see Star Wars: The Force Awakens as he had planned to do for months. After hearing Professor Proton's words, Sheldon decides to prove his devotion to Amy by finally having sex with her, as she has been hoping he will do for the past few years. Some time later, after Sheldon takes Amy to see The Force Awakens, Jeffries returns to Sheldon to ask him about his first sexual experience with her, but Sheldon thinks Jeffries is talking about the film and tells him he enjoyed it.
For his role as Jeffries, in 2013, Newhart won his first Primetime Emmy Award.
Barry Kripke, PhD (John Ross Bowie) is a coworker of the guys, who frequently clashes with Sheldon. He works in plasma physics. Kripke has a case of rhotacism in which he pronounces the letters "R" and "L" as "W" in much the same way as Elmer Fudd from Looney Tunes and Gilda Radner in her "Baba Wawa" sketches. In season five, Kripke purchases an iPhone with voice-recognition technology, but due to his rhotacism, the device is incapable of understanding his verbal "wequests", prompting him to deride it as ineffective. Until season seven, whether he was aware of his impediment was never clarified.
In his first appearance, he pits his robot, the Kripke Krippler, against the men's robot, M.O.N.T.E., in an unofficial robot fight. The guys lose to Kripke, and M.O.N.T.E. is destroyed. In "The Friendship Algorithm", Sheldon attempts to befriend Kripke to gain access to an open science grid computer to carry out research, but it turns out to be futile, as Kripke has no control over the computer's usage time, and Sheldon consequently expels Kripke from the group. Kripke continues his antagonism towards Sheldon in "The Vengeance Formulation" by pulling a prank on Sheldon when he is a guest on NPR's Science Friday by putting a helium tank in his office. In "The Toast Derivation", Sheldon tries to befriend Kripke and a few others when he is feeling excluded by his friends, but Kripke and the others all connect in a manner that Sheldon finds 'wrong' and leaves.
In season five, Kripke and Sheldon battle over a retired professor's office, but after a long sports contest, Kripke loses to Sheldon. He is later invited to Howard's bachelor party, where he uses his turn toasting to complain that no strippers are at the party.
In season six, Kripke and Sheldon are forced to work together on a proposal involving fusion reactors, which involves an exchange of work. Sheldon is shocked to discover that Kripke's work is more advanced than his own, and he tearfully admits as much to Amy. Kripke assumes that Sheldon is in a sexual relationship with Amy, and that has led to his work quality declining, and Sheldon does not deny it (although this appears to be just a convenient excuse since his work is inferior to Kripke's). Kripke also battles for tenure alongside Sheldon, Leonard, and Raj.
In season seven, Kripke ridicules Sheldon when his discovery of a new element was false, nicknaming Sheldon "The Retractor".
In season 9, Kripke calls Sheldon and Amy's live show Fun with Flags along with Raj and directs the conversation to irrelevant questions to the show. He also uses the show to call out for potential romantic relationships and asked Amy questions about her love life when Sheldon and she broke up. He attends Sheldon's birthday party and constantly hits on Leonard's mother, Beverly.
Professor Bertram Kibbler (Brian Posehn) is an employee of the Caltech geology department who met Amy while she was working there in her own lab in "The Occupation Recalibration". Bert keeps bringing Amy interesting and beautiful rocks every day. It takes Howard and Raj to point out that he likes her. Bert then asks her to a rock and mineral show which she declines because she has a boyfriend. When Bert starts talking about how no one wants to do anything with him, Amy in a fit of sympathy agrees to go. Unfortunately, Bert is taking it as a date. Raj and Howard offer to talk to him to let him down easy and they end up going with him to the show.
Bert first appeared in "The Contractual Obligation Implementation" where he is socially awkward and Raj meets just prior to a library date with Lucy. He tried to crash their date and Raj sent him away. His name wasn't mentioned in the plot, but the closing credits gave the name Bert.
In "The Dependence Transcendence", Bert hosts a Caltech party that only Amy and Penny attend. He feels that Amy is the coolest girl at work and that Sheldon is cool only because he is dating Amy. As the girls are leaving, he tells them that he is in love with both of them.
He reappeared in "The Fetal Kick Catalyst" as a guest at Sheldon and Amy's brunch in their apartment. He mentions that a girlfriend he was living with once moved out and took all his best rocks.
In "The Geology Elevation", Bert wins the McArthur Fellowship Grant from Caltech, which makes Sheldon jealous. When he is finally confronted by Sheldon over the latter's jealousy, Bert points out that the theoretical physicist has his friends and Amy to be happy about. He later goes with Sheldon to The Ellen DeGeneres Show to make peace with him. His full name is shown in his award.
Dr. Beverly Hofstadter (Christine Baranski) is Leonard's narcissistic, unloving, and overly analytical mother. Beverly is a neuroscientist, as well as a psychiatrist. She is Sheldon's female equivalent regarding neurotically strict speech patterns, lack of social conventions, and attention to detail. The only reason she had children was to use them as personal experiments for her research, so was very distant and unmotherly towards Leonard. Beverly mentions that Leonard's brother and sister are more successful in their respective fields than he is, though she respects this enough to keep contact with them (unlike Leonard); she is not proud, since they are not her accomplishments. One of her famous books is Needy Baby, Greedy Baby, in which she describes an infant's (Leonard's) unrealistic nurturing expectations. Sheldon often refers to her books when frustrated with Leonard's attempt to obtain some of his personal needs in their relationship. Leonard sees his mother as cold and not nurturing, even telling Penny that one time he made a hugging machine just to get hugs (and that his father asked to borrow it). The lack of Beverly's maternal feelings and actions toward Leonard has led to him being called the "King of Foreplay" because of his obsessive need to please.
In her first visit, Beverly makes Penny cry by delving into her history with her father (driving her, and later Leonard, to the bottle), and considers Raj and Howard's relationship to be an "ersatz homosexual marriage". After bonding with Sheldon, Beverly sings Journey's "Any Way You Want It" with him on karaoke. In season three, Beverly comes to visit at Christmas, where she later gets drunk with Penny at the Cheesecake Factory, gropes a busboy, and then goes to Del Taco and returns to Leonard and Sheldon's apartment where she kisses Sheldon while intoxicated.
In season five, Sheldon calls Beverly on Skype to help Leonard following Penny and Raj's hook-up, and she tells Leonard to "buck up, sissypants...If you need any more help from me, my books are available on Amazon".
In season seven, Leonard uses Beverly's latest book to get sympathy from Penny. However, Bernadette finds out from Howard what Leonard is doing and tells Penny, who calls Beverly, leaving a briefly pantsless Leonard to chat with his mother on Skype. Leonard later informs his mother of his engagement to Penny, but she reveals that Sheldon already told her. She decides to accept his relationship with Penny because Sheldon likes her.
In season eight, Beverly comes to visit to see Leonard receive an award, where she meets Sheldon's mother, Mary. The mothers get into an argument after Beverly describes Mary's religious beliefs as a superstition, and Mary, in turn, berates Beverly for her cold and distant attitude towards Leonard. Beverly later reflects with Sheldon that perhaps other ways exist to raise children than she did. The mothers make up and Beverly tries to show affection by hugging Leonard, though it is awkward for both of them.
In season nine, she attends Sheldon's birthday party and enjoys getting hit on by Barry Kripke. Later, she admits that she feels insulted about not being invited to her son's wedding and accepts a wedding redo. She is not happy about Leonard's father coming since their marriage ended because he had an affair. The two of them end up arguing all night and Beverly once again fights with Mary. At the ceremony, she agrees with her ex-husband that Leonard was the best thing to come out of their relationship.
Claire (Alessandra Torresani) is a writer for a children's science fiction series whom Raj and Howard meet in the comic book store. Claire wants to collaborate with Raj on the science portions of a movie script. She is only interested in his help, while Raj has visions of marriage and children. After Raj breaks up with Emily, he calls Claire on Valentine's Day; however, she just got back together with her boyfriend. She also calls Raj an 'ass' for breaking up with Emily just before Valentine's Day, although later she asks Raj out for a drink when she again breaks up with her boyfriend. She continue to date Raj, but they keep their relationship casual with no commitment.
Dave Gibbs (Stephen Merchant) is a tall British man who dates Amy after she breaks up with Sheldon. Dave is recently divorced because his wife left him for a French chef. When her friends try to find Amy a date on a dating mobile app, they see a text message from Dave, and Amy reveals they have already gone out together for coffee. Sheldon is about to propose to Amy for closure when he sees Amy and Dave share a kiss after their date. When Amy and Dave later go out again for dinner (while Penny, Bernadette, and a reluctant Leonard spy on them), he shows an unusual fascination with Sheldon, which discomforts Amy. He considers Sheldon and Leonard to be physics superstars and idolizes them. He later gets an opportunity to meet Leonard and shake his hand after Leonard has to relieve himself while spying on their date, and with great excitement, Dave announces that he will never again wash his hand. After Sheldon rejects Amy's proposal that they be a couple again, Amy once again tries dinner with Dave, who goes out of his way to refrain from mentioning Sheldon, knowing how uncomfortable that subject made her previously. Sheldon then suddenly arrives and professes his love to Amy, and the two mend their relationship, leaving Dave to awkwardly excuse himself and invite Sheldon to meet again on his way out.
Dr. Eric Gablehauser
Dr. Eric Gablehauser (Mark Harelik) is the head of the Physics Department, the group's boss at the university. When he first starts working at the university, he fires Sheldon after Sheldon insults his intelligence. Eventually, Sheldon is rehired because Gablehauser became romantically interested in Sheldon's mother. Gablehauser later hosts the Physics Bowl, and gives Raj attention when he is featured in a People article for discovering a planet.
Dr. V.M. Koothrappali
Dr. V.M. Koothrappali (Brian George) is Raj's father in India. His wife and he communicate with their son via Skype and constantly try to arrange dates for him. They want their son to marry a woman of Indian descent, and give them grandchildren. Although in many episodes Raj mentions that he grew up in poverty, his friends remind him of the contrary by pointing out that Dr. Koothrappali is a gynecologist, drives a Bentley, and employs servants. In one such episode, Raj replies that the Bentley is leased and there are only four servants in his family home, two of whom are children. Dr. Koothrappali enjoys Doogie Howser reruns, which are apparently new to India. Other than Raj, he also maintains contact with their daughter Priya via Skype whenever she is out of India. In season four, Dr. Koothrappali becomes angry with Leonard when he learns that his daughter, Priya, is dating Leonard without his knowledge. According to Sheldon, the Koothrappalis are "Richie Rich rich".
The Koothrappalis go through a rough patch in their marriage, culminating in them separating in season eight around the time of their 40th wedding anniversary, with Dr. Koothrappali moving out and Mrs. Koothrappali setting fire to his car. Dr. Koothrappali makes his first physical appearance on the show in season eight, visiting the main characters at a Christmas party after the divorce. Dr. Koothrappali later cuts off Raj financially when he buys a helicopter drone, but Raj manages to turn the situation around by pitting his father against his mother. He also asks his father to give advice to a nervous, pregnant Bernadette.
Mrs. Koothrappali (Alice Amter) is Raj's mother in India. Mrs. Koothrappali is especially worried that, despite Raj being old enough to marry, the closest they have to a daughter-in-law is "that little Jewish boy Howard". She enjoys Doogie Howser reruns, which are apparently new to India. Other than Raj, she also maintains contact with Priya via video chat whenever she is out of India. She also had some anger towards Leonard when it became apparent that he was dating Priya without her knowledge.
Following their divorce in season eight, Raj mentions that his mother bought the book Eat Pray Love and used it to set fire to her ex-husband's car. Mrs. Koothrappali makes her first physical appearance on the show in "The Graduation Transmission", where Raj uses the Koothrappali's divorce to get money from her when his father cuts him off financially, and pits her against her ex-husband.
Janine Davis (Regina King) is a human resources representative working at the university. She first appears in season six after Sheldon offends his assistant Alex while trying to solve a problem regarding her crush on Leonard, leaving Mrs. Davis to handle Alex's sexual-harassment complaint against Sheldon. Sheldon tells Mrs. Davis his belief that she, like all women, is a "slave" to her sexual "urges", unaware that his remarks are racist and sexist. In his attempt at defense, Sheldon rats out what he considers similar behavior by Raj, Howard, and Leonard, prompting Mrs. Davis to summon them to her office, as well. Mrs. Davis later serves on the tenure committee that is considering Raj, Sheldon, Leonard, and Kripke for a vacant tenured position, causing each of them to try to win her favor. Sheldon, in his effort to earn the position, offends her once again by giving her the box set for the TV series Roots. Despite this, Sheldon is still short-listed for tenure, even if he offends Mrs. Davis yet again by giving her what he thinks is a traditional handshake originating in African American communities.
In season seven, Mrs. Davis says that she is recently divorced because her husband cheated on her, and Raj attempts to strike up a conversation with her at a university mixer. After offending her, Raj apologizes, and the two talk about their troubles. Following the mixer, while Raj is insistent that Mrs. Davis and he had a "moment", Howard disagrees.
Kurt (Brian Patrick Wade) is a tall, intimidating bodybuilder and Penny's ex-boyfriend at the beginning of the series. In the pilot, Penny reveals that he cheated on her, and Leonard and Sheldon later go to his apartment and attempt to retrieve Penny's television set from him, but they were unsuccessful and Kurt apparently found them so irritating that they returned home without pants. Kurt is invited to Penny's Halloween party, for which he dresses up as a caveman, where he has yet another run in with Leonard, who is dressed as a hobbit. After Kurt sees Penny kissing Leonard, Leonard victoriously shouts to him, "That's right, you saw what you saw. That's how we roll in The Shire!"
In season two, Penny experiences financial difficulties. After Leonard learns about Penny's problems, he tries to help her cut expenses, and in the process learns that Kurt owes her $1800. Without telling Penny, Leonard and the guys go to Kurt's place to collect the money, but they are unsuccessful, and Kurt writes "I owe Penny $1800 ~ Kurt" on Leonard's forehead in indelible ink. Later, Penny reveals that Kurt visited her the next morning and paid her, in turn. However, much to Leonard's disappointment, Kurt never told Penny about Leonard's visit, and instead took the opportunity to ask Penny out for dinner.
Lucy (Kate Micucci) shows up at a party at the comic-book store on Valentine's Day at an event for people who had no dates. She has social anxiety issues, which allows Raj and her to connect. On their first date, she becomes so nervous that she goes to the bathroom and climbs out of the window to get away. Raj falls apart and refuses to leave his apartment; however, after apologizing to him, Lucy and Raj start afresh. Raj and Lucy have a date at a library and communicate by text messages, and at the end, she offers to kiss him, but chickens out at the last second. In "The Bon Voyage Reaction", Raj tries to introduce Lucy to the rest of the gang. Lucy becomes nervous at the thought of meeting so many new people that Raj suggests she meet just one of his friends first, to which she agrees. Lucy meets Amy, but Raj pressures Lucy and upsets her. Lucy breaks up with Raj by an email, and afterwards a heartbroken Raj finally speaks to Penny without having consumed alcohol.
In season seven, Penny meets Lucy at the Cheesecake Factory and confronts her about her breakup with Raj, and later Lucy arranges to meet with Raj to apologize to him for breaking up with him via email. This leads Raj to believe the two of them will get back together, but Lucy tells him she is with someone else. Lucy is later mentioned in "The Indecision Amalgamation". Raj gets a message from her about getting back together, but after finding out that he is also dating Emily, Lucy rebuffs Raj.
Mary Cooper (Laurie Metcalf) is Sheldon's loving, caring and religious mother and a devout Christian from Texas. She has two other children besides Sheldon; Sheldon's twin sister, Missy, and his brother George, who is shown via a family tree to be three or four years older than Sheldon and Missy. Her husband, a rambunctious alcoholic, was also named George, but he died when Sheldon was still a boy. To Mary's relief, her other children do not share Sheldon's hyperintelligence, once commenting to Leonard, "I thank the good Lord my other kids are as dumb as soup". Mary herself is not intellectual, but is quite wise, though she is prone to making insensitive comments such as calling Japanese words "kung fu letters" at a sushi restaurant, or calling Native Americans ""our" Indians". Despite her extreme religious views, she is tolerant of other faiths, as evidenced by her serving Raj a dish of chicken after making sure it was not something Indians considered "magical". She also struggles to follow her beliefs sometimes. She is a loving mother and has been able to control Sheldon, with Leonard describing her as Sheldon's "Kryptonite" to Penny. When Penny and Sheldon engaged in a fierce dispute, Penny called for Mary's help, who subsequently called and scolded Sheldon about his actions. Leonard wishes his mother were as loving as Sheldon's mother, although Sheldon himself appears at best ambivalent about her parenting. Sheldon once mentions that Mary once hit him with a Bible because he refused to eat his Brussels sprouts. She apparently thought something was wrong with her son while he was growing up, as Sheldon will often remark after someone calls him crazy, "I'm not crazy; my mother had me tested!" Though Mary confirms this, she wishes that she had taken him to Houston for further testing.
In her first visit, Leonard calls Mary to intervene when Sheldon is fired for being disrespectful to his new boss, Dr. Gablehauser. She manages to convince Sheldon to apologize and he gets his job back, and she becomes romantically interested in him. When Sheldon asks, "Is Dr. Gablehauser going to be my new daddy?", Mary replies, "We'll see".
In season three, after returning from the North Pole, Sheldon returns to his home in Texas after the other guys tampered with his research. Mary calls the guys and they come to fetch Sheldon. Sheldon initially refuses to leave, although after an argument with his mother about evolution, Sheldon decides to leave. Mary is again called to intervene in "The Zazzy Substitution" when Sheldon buys a clowder of cats after his "break-up" with Amy. Mary uses reverse psychology to get Sheldon to reconcile with Amy. She again comes to visit in "The Rhinitis Revelation" before going on a Christian cruise, and Sheldon becomes jealous when she would rather go sightseeing with the rest of the gang instead of cooking for him and attending physics lectures.
In season seven, Sheldon returns to Texas to assist Mary in the birth of his twin sister's baby. Howard later takes Sheldon to see Mary after visiting NASA headquarters, but flees after Sheldon sees her having sex. Sheldon returns and gets into an argument, and is sent to his room. After Howard tells him about how he drove away an admirer of his mother, Sheldon apologizes to Mary, saying that he will accept her choices on the outside while condemning her internally for being a hypocrite by going against her religious beliefs. Mary forgives him.
In season 9, Sheldon calls her to tell her that Amy broke up with him and that he would like to send the family heirloom engagement ring back, but she advised him to hold on to it, just in case.
Mike Rostenkowski (Casey Sander), Bernadette's father is retired police officer. Mike has a rough and rude personality and often bullies others, although he deeply loves his daughter, calling Bernadette his "little girl". Before he is first seen, Bernadette establishes his personality by telling Howard a long list of subjects he cannot discuss with Mike, including Jimmy Carter, foreigners, homosexuals, and even Howard's Jewish identity.
In his first appearance, he tells Howard, while he does not think highly of him, his respect for Howard grew when he learned he had been selected to become an astronaut. However, after Howard expresses his fears about not returning from space, Mike believes Bernadette will have no trouble finding another suitable bachelor if Howard dies in space, much to the future astronaut's dismay. In the season-five finale, Mike attends Howard and Bernadette's wedding and walks his daughter down the aisle.
In season six, Howard and Mike are pushed into a fishing weekend by their spouses, but neither man really wants to go. Mike suggests that they go to a casino to shoot craps. Having earned more of Mike's respect, Howard is now able to address him as "Mike" rather than "Sir".
In season seven, because his wife is out of town, Bernadette gets Mike to come to Howard's mother's house for Thanksgiving dinner, though he would rather watch football at home. Since Sheldon understands football from growing up with his father, Mike and Sheldon bond and drink heavily together. After insulting Howard, Mike later tells Howard that he is not "all that bad", but implies that this statement is untrue when he comments on his drunken state. In "The Table Polarization", Mike, Bernadette, Raj, and Mike Massimino (via Skype) perform an intervention on Howard's decision to go back to space. Mike mentions that Howard asked him to shoot him in the foot to avoid the first space mission, which Howard insists was a joke.
In season nine, he comes to Howard and Bernadette's house since they are talking about renovating and he comes over to help. While Howard and he are busy together, Mike asks Howard why he does not want kids, but Howard sets things straight when he tells Mike that it is actually Bernadette who does not want kids and she causes the dust avalanche on both of them. Bernadette had lied to her father about not wanting kids because she did not want to disappoint him. Mike says he does not want to see her miss out since raising children was the most rewarding experience of his life, but Bernadette points out that her mother did everything while all Mike did was come home from work, sit on the couch, and drink beer, which is unrelated to raising kids. Defeated, Mike leaves, saying this is really a conversation for husband and wife and tells Bernadette if her mother asks, he was at the Wolowitz house until 10. He briefly comes back into the kitchen and sees Howard and Bernadette doing "dirty talk" and then disappears again through the doorway.
President Siebert (Joshua Malina), the president of the California Institute of Technology, is married and has a teenaged daughter. Siebert is often mentioned by Sheldon. A recurring gag is that Sheldon often calls or visits Siebert in the middle of the night to get information, and usually gets a bitter response. Siebert sends Sheldon on the expedition to the North Pole at the end of season two.
In his first appearance, Siebert organizes a get-together with wealthy donors and faculty members to garner funds for the university's programs. He urges the guys to make an appearance to schmooze the donors so they will give to their projects. Seeing Sheldon's behavior toward the donors, he asks that he not appear again.
In season five, Siebert forces Sheldon to use up his vacation days, and when Sheldon and Kripke battle for an office at the university, they interrupt Siebert at a urinal to get him to make a decision on who gets the office.
Priya Koothrappali (Aarti Mann) is Raj's younger sister. Having graduated at the top of her class at Cambridge University (where like Leonard, her roommate was from Texas), she is one of the lead attorneys at India's biggest car company.
In her first appearance, Priya has a two-day layover and comes to visit her brother. Leonard and she are revealed to have had a relationship five years earlier (without her parents or Raj's knowledge) when she was in town, and that Leonard had been much more invested in the relationship than she was. Leonard even offered to relocate to New Delhi to be with her. He tries to keep their relationship a secret from the guys, but Sheldon discovers Priya in their apartment after spending the night together. Leonard eventually reveals to the rest of the guys that he spent the night with Priya, as he is unable to bear the burden of keeping the secret. Priya returns to Los Angeles for business in "The Cohabitation Formulation", and Leonard and she decide to try to have a real relationship, over the protests of Raj. It upsets Raj to know that his sister is having sex with one of his best friends (sometimes in Raj's bed). Priya soon becomes jealous of Penny and asks Leonard to stop spending time with her, which causes Penny, Amy, and Bernadette to dislike her. Priya contests Sheldon's roommate agreement with Leonard, employing her legal background to deny Sheldon power over him, but Sheldon drafts a new agreement and blackmails Leonard into signing it by threatening to tell Priya's parents she is dating Leonard, which she cannot bear because they want her to date an Indian (although her father would accept her dating Howard due to his belief that Jews are financially savvy and do not drink much). At the end of season four, it she is moving back to India and has not told Leonard, which Leonard interprets as the end of their relationship, leading him to expose their secret relationship to her parents.
In season five, although they are separated, with Priya moving back a month earlier than expected, they nonetheless remain in touch via Skype, having a cyber-dinner/breakfast date. At Howard's suggestion, Leonard and Priya also attempt to simulate intercourse, but when it comes time for her to remove her clothing, Leonard's screen freezes up because of his exceeding the bandwidth. Leonard later manages to get cue cards to help himself with a seduction speech to Priya, but while delivering it to her, he inadvertently reveals to her parents that they are cyber-dating (they were in the same room as she when he called), with them acidly asking for him to continue with the speech. In "The Good Guy Fluctiation", Leonard tells Priya about his attraction to a comic-book enthusiast named Alice; Priya admits to cheating on him with her former boyfriend, which visibly upsets Leonard. When Priya states that they both messed up a little, Leonard retorts that while he messed up a little, she messed up a lot. She claims that it is not a competition, to which he replies that it is and that she won and he switches off his laptop. In the next episode, Leonard refers to himself as single, indicating that Priya and he have broken up as a result of their last conversation.
Stephanie Barnett, MD (Sara Rue) is a doctor and highly distinguished surgical resident at Fremont Memorial. She did her medical internship at Lawrence Memorial in Galveston, Texas, which is where Sheldon was born.
Stephanie is picked up in a bar by Howard, but when his plan to let her drive the Mars rover fails, she leaves Howard's office with Leonard and the two immediately show an attraction to each other. Howard becomes angry when he finds out, but he soon changes his mind when Stephanie sets Howard up with one of her friends. Sheldon begins meddling in Stephanie and Leonard's new relationship in an effort to strengthen it, since in his eyes, Stephanie is the only "tolerable" mate Leonard has had. After several weeks of dating, Stephanie officially begins living with Leonard in Sheldon and his apartment (although Leonard does not realize this until Sheldon and Penny point it out to him). Stephanie uses sex to manipulate Leonard when he tries to slow down the relationship. After much hesitation, Leonard texts her at the university and tells her to move out because he is uncomfortable with the pace of the relationship, although he apparently leaves to have sex with her. They are not seen breaking up on camera and she is neither seen nor mentioned again.
Series cocreator Bill Prady said of the relationship: "Stephanie was a chance for Leonard to learn that just because someone loves you, doesn't mean you'll love them back".
Wil Wheaton plays a fictionalized version of himself. Will played Wesley Crusher in Star Trek: The Next Generation, whom Sheldon idolized as a child, and became a "mortal enemy" of his after an incident that occurred in 1995, when Sheldon was devastated because Wheaton failed to show up at a fan convention that Sheldon had taken a 10-hour bus ride to attend.
In season three, Sheldon enters a trading-card game tournament (Mystic Warlords of Ka'a) to confront Wheaton. When Sheldon is about to defeat his hated rival, Wheaton tells him that he missed the 1995 "Dixie Trek" convention because his grandmother had died. In sympathy, Sheldon deliberately messes up his next move to benefit Wheaton. Wheaton reveals that he lied about his grandmother and then wins the game by taking advantage of the opening Sheldon leaves him. Wheaton later plays in a bowling match against Sheldon, where he uses gamesmanship again, breaking up Leonard and Penny to win.
In season four, Wheaton appears at a special screening of Raiders of the Lost Ark and uses his celebrity status to go to the front of a long line. This antagonizes Sheldon, who derides him as the "Jar Jar Binks of the Star Trek universe". Wheaton and his entourage effectively prevented Sheldon and his friends from attending the screening when the last seat in the theater went to the person directly in front of them in line. Ultimately, Sheldon steals the reels containing the prints of the film and Wheaton leads a mob of angry audience members after Sheldon.
In season five, Wheaton invites the guys to a party at his house and appears to be genuinely sorry for his past actions, giving Sheldon a signed Wesley Crusher action figure as a way of thanking Sheldon for being a fan. His gesture is met with a renunciation of his "mortal enemy" status and a hug from Sheldon. Wheaton is invited to Howard's bachelor party, where he films the toasts and uploads them to the Internet, getting Howard and Raj in trouble with Bernadette.
In season six, Wheaton appears in a Star Trek-themed episode of Sheldon's web-series Fun with Flags and insults Amy after she criticizes his performance. After Sheldon ditches Amy to spend time with Wheaton, Amy becomes angry with Sheldon, who later unknowingly becomes intoxicated and almost starts a fist fight with Wheaton, demanding an apology.
In season seven, Amy calls Wheaton to cheer up Sheldon after Leonard disproves Sheldon's scientific discovery. Wheaton later gives Penny advice when she wrestles with whether to take a role in the sequel to a low-budget horror film in which she once starred. After taking the role, Penny finds that Wheaton is also appearing in the film. Wheaton and Penny are both fired from the film after an argument with the director.
In season eight, Wheaton invites Penny to appear on his podcast, where she talks about her acting career. Leonard and Penny get into an argument on air, much to the amusement of Wheaton.
In season nine, Wheaton introduces Adam Nimoy, son of Mr. Spock portrayer Leonard Nimoy, to Sheldon, for Sheldon to be interviewed for a documentary that Adam Nimoy is producing about Spock and his impact on popular culture. In a later episode, Wheaton fills in for Sheldon when he joins Leonard, Howard, and Raj for the midnight premiere of Star Wars: The Force Awakens. When he arrives at the theater, the audience immediately jeers him for dressing as Spock, but Wheaton explains that he did so intentionally because he believes the audience should not take whether the film succeeds or fails so seriously. He appears again to celebrate Sheldon's birthday in the show's 200th episode, "The Celebration Experimentation".
Wheaton, LeVar Burton, Leonard Nimoy, Brent Spiner, and George Takei have all appeared on the show, making the Star Trek franchise the most represented franchise on the show in terms of guest appearances.
Mrs. Debbie Wolowitz (voiced by Carol Ann Susi) is Howard's over-protective, controlling, belittling, but loving and caring stereotypical Jewish mother. The character is not shown on-screen with the exception of one episode in season six in which her body, but not face is briefly shown. Mrs. Wolowitz's raspy voice is heard usually in scenes at her house, which she shares with Howard. She talks to Howard always by yelling at him from another room, which results in awkward conversations with Howard yelling back at her. In a flashback, when the guys hear her yelling in a masculine voice, they ask Howard if that was his father, to which Howard replies, "If she grows any more hair on her face, yes".
Mrs. Wolowitz is also described as being enormously obese and wearing a wig and painted-on eyebrows. She seems to be oblivious to Howard's work as an engineer and treats him as though he is still a child. She frequently refers to Leonard, Sheldon, and Raj as Howard's "little friends" (as in, "I made some cookies and Hawaiian Punch for you and your little friends!") and often refers to Caltech as a "school". Howard often makes references to his mother as being a "crazy old lady", but deep down, he is deeply attached to his mother and quite happy to be treated like a child. However, in one episode, Howard attempts to move out after a fight with his mother, but has serious delusions that she is trying to force him to stay, when in actuality, she is urging him to move out, which leads to him staying. She is also noted to be on hormone replacement therapy, and she wants to "play doctor" with Raj when he asks to sleep over.
When Howard reveals that he has proposed to Bernadette and she has accepted, she collapses, causing Howard to panic and dislocate his shoulder while trying to barge into the bathroom to help her. The timing leads him to believe that she may not like Bernadette (who is Catholic) and has suffered a heart attack from the shock of the news. In actuality, her fainting was due to food poisoning from the food she ate while meeting Bernadette and was completely unrelated to Howard's news. She has graciously accepted Bernadette as her daughter-in-law and is impressed by Bernadette's doctorate.
In season seven, Mrs. Wolowitz breaks her leg and keeps driving away the nurses hired to care for her. Even Penny rejects the job of caregiver after a very short time. After Stuart's comic-book store burns down, Howard invites Stuart to work for Mrs. Wolowitz; however, they develop a strange relationship, much to Howard's annoyance.
Following the death of Carol Ann Susi, producers decided that Mrs. Wolowitz would die in the season-eight episode "The Comic Book Store Regeneration". Howard receives a phone call while in the store from his aunt Gladys in Florida, whom Mrs. Wolowitz had gone to visit. She says that Mrs. Wolowitz lay down for a nap and never woke up afterwards. Howard and Bernadette then move back into Howard's old home, and Stuart decides to stay with them. He eventually moves out halfway through season nine.
Zack Johnson (Brian Thomas Smith) is a dim-witted but friendly beefcake-type whom Penny dates on and off after her second break-up with Leonard. Zack works as the menu designer for restaurants that are owned by or which have hired his father's company to design their menus.
In season three, Leonard invites Penny and Zack up to the roof to watch his experiment of bouncing laser beams off the moon; Zack misunderstands what is happening, and thinks they are going to blow up the moon. Later, Penny and he go out to a party, but she is so put off at how stupid he is that she runs back to Leonard for sex, saying he ruined her ability to tolerate stupid men.
In season four, Zack runs into Penny, Bernadette, and Amy at a restaurant, and Amy becomes physically attracted to him. Sheldon arranges a meeting between Zack and Amy to satisfy her "urges", but his lack of intellect immediately discourages Amy. Unlike Penny's other boyfriends, Zack considers Leonard, Howard, Sheldon, and Raj (whom he calls "the science dudes") to be "cool" and makes a genuine effort to get to know them, even treating them as his friends. After accompanying them on a trip to the comic-book store (where Raj and he bond over a mutual fondness for Archie comics), he enters a New Year's Eve costume party with them, portraying Superman in their version of the Justice League. In "The Toast Derivation", Sheldon invites him over for a party, where he bonds with Kripke and Stuart, as well.
In season seven, he was unknowingly married to Penny for three years after the two were intoxicated and attended a wedding chapel in Las Vegas (they were not aware that weddings in Las Vegas are real). Penny invites him to Mrs. Wolowitz's house for Thanksgiving, where they get the marriage annulled.
In season nine, episode 22, Zack's observations indicate he is, in some respects, more observant and smarter than "the science dudes".
- Althea (Vernee Watson) who makes an appearance in the pilot episode as an attendant at a sperm bank, and then makes three appearances as an emergency room nurse in "The Peanut Reaction" episode (season one) and "The Robotic Manipulation" episode (season four), and as a ward nurse in "The Werewolf Transformation" (season five). The name of the character is never used on the show, but is included on her name tag and in the credits. She is also the only character besides Leonard and Sheldon to be carried over from the original unaired pilot.
- Captain Sweatpants (Ian Scott Rudolph) and Lonely Larry (Owen Thayer) are sometimes seen at the guys' favorite comic-book store. Captain Sweatpants is a middle-aged bald man who wears grey sweatpants and a City of Heroes T-shirt. Lonely Larry wears a brown suit and is extremely thin. They are also friends with Wil Wheaton. Both attend Howard's bachelor party.
- Cheryl (Erin Allin O'Reilly), apparently a friend of one of Penny's friends (because she may not know many of the people at the party), who attends Penny's Halloween party in season one, episode six, "The Middle Earth Paradigm". Cheryl is the very talkative, short-haired brunette dressed as a ladybug, who shows interest in Raj. She is not identified within the episode, but her name is listed in the credits. She appears again as a Cheesecake Factory waitress in "The Pancake Batter Anomaly".
- Dale (Josh Brener) works as a replacement for Stuart at the comic-book store when he goes on a date with Amy in "The Flaming Spittoon Acquisition", and also appears in "The Tangible Affection Proof" at Stuart's Valentine's Day party.
- Dan (Stephen Root) is Bernadette and later Penny's boss. Bernadette arranges an interview for Penny with Dan. The interview goes terribly, but Dan eventually hires Penny when they bond over their shared fear of Bernadette.
- Dimitri Rezinov (Pasha Lychnikoff) is a Russian cosmonaut who is Howard's colleague on his Soyuz mission to the International Space Station.
- Wyatt (Keith Carradine) is Penny's father. He comes to visit, and Penny feels compelled to involve Leonard in a ruse to make him think they are still together. (Wyatt reveals that he favors the successful Leonard over Penny's other "loser" boyfriends.) When the plot is revealed, he gets mad at Penny and feigns anger at Leonard, but after Penny leaves the room, he encourages Leonard to keep trying to get Penny back (because he wants his grandchildren to "grow up in a house without wheels"). Strangely, though, in the first episode where Leonard's mother visits, Penny tearfully reveals her rough childhood, saying that Wyatt wanted a boy and even called her "slugger" until she reached puberty. Wyatt appears again in season nine, when Penny finally plucks up the courage to tell her father she eloped with Leonard. Wyatt is happy, but he also confesses that he killed Penny's pet pig a year ago.
- Blake Anderson as Trevor is an individual with whom Sheldon has conflict when he cuts in front of them at the movie theatre.
- Lewis Black as Professor Crawley is a former entomologist at the university with whom Sheldon, Howard, and Raj confer about a cricket.
- Eliza Dushku as Agent Angela Page is an FBI agent who was asked to interview Howard's acquaintances to determine his eligibility to use the Defense Department Laser Equipped Surveillance Satellite team.
- Judy Greer as Dr. Elizabeth Plimpton is an acquaintance of Sheldon's, who upon visiting, engages in flirtatiously sexual activities with Leonard, Howard, and Raj.
- James Hong as Chen is maitre d' at a Chinese restaurant.
- Jane Kaczmarek as Dr. Gallo is a therapist to whom Penny attempts to sell products. She winds up conducting therapy on both Penny and Leonard.
- Jack McBrayer as Randall is Penny's brother.
- Danica McKellar as Abby is a girl Raj briefly dates.
- Annie O'Donnell is Mrs. Fowler, Amy's mother, in "The Desperation Emanation".
- DJ Qualls as Toby Loobenfeld is a scientist at CalTech whom Sheldon hires to play his fictional drug-addicted cousin Leo.
- Michael Rapaport as Kenny Fitzgerald is an intelligent thief who sells stolen liquid helium to Leonard and Sheldon.
- Katey Sagal is Susan, Penny's mother
- Octavia Spencer as Octavia is a DMV worker.
- Yeardley Smith as Sandy is an employee at the job bank when Sheldon seeks a part-time job to clear his head.
- June Squibb as Meemaw (real name Constance) is Sheldon's grandmother from Texas. She takes an immediate dislike to Amy upon meeting her.
- Billy Bob Thornton as Dr. Oliver Lorvis is a medical doctor to whom Penny sells products. He misreads her flirtations and locks the guys in his basement while he goes to woo her (as well as Amy and Bernadette).
- Jessica Walter as Mrs. Latham is a donor for the university who goes out on two dates with Leonard.
Guest stars appearing as themselves
A number of Hollywood celebrities and famous scientists and engineers have made appearances as themselves.
- Buzz Aldrin: "The Holographic Excitation"
- LeVar Burton: "The Toast Derivation", "The Habitation Configuration", "The Champagne Reflection"
- Nathan Fillion: "The Comic Book Store Regeneration"
- Carrie Fisher: "The Convention Conundrum"
- Ira Flatow: "The Vengeance Formulation", "The Discovery Dissipation"
- Summer Glau: "The Terminator Decoupling"
- Brian Greene: "The Herb Garden Germination"
- Stephen Hawking: "The Hawking Excitation", "The Extract Obliteration", "The Relationship Diremption", "The Troll Manifestation", "The Celebration Experimentation", "The Geology Elevation"
- James Earl Jones: "The Convention Conundrum"
- Stan Lee: "The Excelsior Acquisition"
- Howie Mandel: "The Re-Entry Minimization"
- Mike Massimino: "The Friendship Contraction", "The Countdown Reflection", "The Decoupling Fluctuation", "The Re-Entry Minimization", "The First Pitch Insufficiency"
- Elon Musk: "The Platonic Permutation"
- Adam Nimoy: "The Spock Resonance"
- Bill Nye: "The Proton Displacement"
- Katee Sackhoff: "The Vengeance Formulation", "The Hot Troll Deviation"
- Charlie Sheen: "The Griffin Equivalency"
- Kevin Smith: "The Fortification Implementation"
- Brent Spiner: "The Russian Rocket Reaction"
- George Takei: "The Hot Troll Deviation"
- Neil deGrasse Tyson: "The Apology Insufficiency"
- Adam West: "The Celebration Experimentation"
- Steve Wozniak: "The Cruciferous Vegetable Amplification"
- Ellen DeGeneres: "The Geology Elevation"
- Although not seen or heard prior to season 4, President Siebert's name is mentioned in season 2.
- "The Middle Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007. 5:18 minutes in.
- "The Boyfriend Complexity". The Big Bang Theory. Season 4. Episode 9. November 18, 2010. 3:30 minutes in.
- "The Apology Insufficiency". The Big Bang Theory. Season 4. Episode 7. November 4, 2010. 12:50 minutes in.
- "The Habitation Configuration". The Big Bang Theory. Season 6. Episode 7. November 8, 2012.
- "The Countdown Reflection". The Big Bang Theory. Season 5. Episode 24. May 10, 2012.
- "The Vegas Renormalization". The Big Bang Theory. Season 2. Episode 21. April 27, 2009. 9:42 minutes in.
- "The Psychic Vortex". The Big Bang Theory. Season 3. Episode 12. January 11, 2010. 8:25 minutes in.
- "The Grasshopper Experiment". The Big Bang Theory. Season 1. Episode 8. November 12, 2007.
- "The Middle Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007.
- Ausiello, Michael (January 23, 2009). "Big Bang Theory scoop: Sara Gilbert taken off contract". Entertainment Weekly. Retrieved November 5, 2016.
- "The Herb Garden Germination". The Big Bang Theory. Season 4. Episode 20. April 7, 2011. 19:08 minutes in.
- "The Stag Convergence". The Big Bang Theory. Season 5. Episode 22. April 26, 2012. 07:41 minutes in. CBS.
- "The Commitment Determination". The Big Bang Theory. Season 8. Episode 24. May 4, 2015. Event occurs at 13:35. CBS. Howard tells Bernadette his middle name, because he wants her to make him move out of his mothers house
- "The Flaming Spittoon Acquisition". The Big Bang Theory. Season 5. Episode 10. November 17, 2011. Event occurs at 11:44. CBS. Sheldon is seen looking at Stuart's Facebook page, where his name is seen to be "Stuart Bloom"
- Andreeva, Nellie (August 22, 2012). "Big Bang Theory 's Kevin Sussman Upped To Regular". Deadline. Retrieved November 5, 2016.
- Ausiello, Michael (October 12, 2015). "The Big Bang Theory: Laura Spencer Promoted to Series Regular". TVLine. Retrieved November 5, 2016.
- Sleasman, MaryAnn. The Big Bang Theory S6E13 blog at Tv.com
- "The Killer Robot Instability". The Big Bang Theory. Season 2. Episode 12. January 12, 2009.
- "The Friendship Algorithm". The Big Bang Theory. Season 2. Episode 13. January 19, 2009.
- "The Vengeance Formulation". The Big Bang Theory. Season 3. Episode 9. November 23, 2009.
- "The Toast Derivation". The Big Bang Theory. Season 4. Episode 17. February 24, 2011.
- "The Bat Jar Conjecture". The Big Bang Theory. Season 1. Episode 13. April 21, 2008.
- "The Griffin Equivalency". The Big Bang Theory. Season 2. Episode 4. October 13, 2008.
- "The Cornhusker Vortex". The Big Bang Theory. Season 3. Episode 6. November 2, 2009.
- "Pilot". The Big Bang Theory. Season 1. Episode 1. September 24, 2007.
- "The Middle-Earth Paradigm". The Big Bang Theory. Season 1. Episode 6. October 29, 2007.
- "The Financial Permeability". The Big Bang Theory. Season 2. Episode 14. February 2, 2009.
- "The Panty Piñata Polarization". The Big Bang Theory. Season 2. Episode 7. November 10, 2008.
- "The Irish Pub Formulation". The Big Bang Theory. Season 4. Episode 6. October 28, 2010. CBS.
- "The Infestation Hypothesis". The Big Bang Theory. Season 5. Episode 2. September 22, 2011. CBS.
- "The Good Guy Fluctuation". The Big Bang Theory. Season 5. Episode 7. October 27, 2011. CBS.
- "The White Asparagus Triangulation". The Big Bang Theory. Season 2. Episode 9. November 24, 2008.
- "The Lizard-Spock Expansion". The Big Bang Theory. Season 2. Episode 8. November 17, 2008.
- "The Vartabedian Conundrum". The Big Bang Theory. Season 2. Episode 10. December 8, 2008.
- "Big Bang Theory: We didn't anticipate how protective the audience would feel about our guys". Variety. May 5, 2009. Retrieved March 7, 2010.
- "The Status Quo Combustion". The Big Bang Theory. Season 7. Episode 24. May 15, 2014. 20:01 minutes in. CBS.
- "The Lunar Excitation". The Big Bang Theory. Season 3. Episode 23. May 24, 2010.
- Prudom, Laura (February 3, 2016). "'Big Bang Theory' 200th Episode: Wil Wheaton, Adam West Among Guest Stars". Variety. Retrieved February 3, 2016.