Booing is an act of showing displeasure for someone or something, generally an entertainer, by loudly yelling boo! (and holding the "oo" sound) or making other noises of disparagement, such as hissing. People may make hand signs at the entertainer, such as the thumbs down sign. If spectators particularly dislike the performance they may also accompany booing by throwing objects (traditionally rotten fruit) onstage, though the objects may not be meant to physically hurt the performer.
Booing performers has a very long history, The first written record comes from ancient Greece. At the annual Festival of Dionysia in Athens, playwrights competed to determine whose tragedy was the best. When the democratic reformer Cleisthenes came to power in the 6th century B.C., audience participation came to be regarded as a civic duty. The audience applauded to show its approval and shouted and whistled to show displeasure. In ancient Rome, jeering was common at the gladiatorial games, where audience participation often determined whether a competitor lived or died.
While people have expressed displeasure publicly since ancient times, the English word boo was first used in the early 19th century to describe the lowing sound that cattle make. Later in the 19th century, the word came to be used to describe the disapproving cry of crowds. Hoot, another onomatopoeic English word, was used as early as 1225 to describe the same phenomenon. (Ancient Greek and Latin both contain words resembling boo that mean "to cry or shout aloud," though there is no known etymological connection to the modern English word.) This practice has in recent times come under criticism. The opinion is often expressed that to boo a bad performance is unkind and demonstrates a lack of sophistication. However, the counterargument goes that the combination of booing and applause help keep the quality of public performance high, by emotionally rewarding the good and punishing the bad. Some ball players who have been booed have stated that booing "spooked" or "bothered" them or their teammates, and that it "affected their performance". Another player stated concerning booing, "It hurts" and another stated, "It frustrates me when the crowd boo". 
- In sports, booing by fans is quite common. They may boo particularly-hated players on the opposing team, or any opposing player when there is an intense rivalry between the teams. Unsportsmanlike behavior is also booed, such as intentionally hitting home team batters in baseball or diving in association football or basketball (where it is a technical foul). Booing of referees or umpires after an unpopular ruling is also common. Booing of expelled players after receiving a second yellow card or a direct red card is also common for many reasons.
- In professional sports, one's own home team, players or coach may be booed due to a poor performance or season.
- A villainous character may also be booed to show a dislike of said character, rather than the acting skills of the thespian portraying him or her. Melodrama performances such as the traditional British Pantomime may encourage it, along with cheering at the hero/heroine.
- In the process of booing, members of the audience sometimes will throw fruit, vegetables or garbage for particularly bad or contemptible performances. In contrast, they might throw flowers for good performances.
- Sonia Smith (May 10, 2006). "Where Do Hecklers Come From?". Slate Magazine.
- Roland Auget (2012). Cruelty and Civilization: The Roman Games. Psychology Press.
- Mark A. (October 8, 2008). "To boo or not to boo?". BBC.
- Aaron Gleeman (October 19, 2012). "Yankee Stadium boos "spooked a lot of guys" according to anonymous Yankee". NBC Sports: Hardball Talk.
- Peter Botte (October 14, 2012). "Yankees' Nick Swisher says fans blamed him for Derek Jeter's injury after he misplayed ball in right field during Saturday's ALCS Game". New York Daily News. "It hurts. Sometimes I’m a sensitive guy and some of the things people say, they get under your skin a little bit. I've been lucky to be here for the past four years, bro. We're not going to go out like this. We’re going to go to Detroit and give everything we’ve got."
- Mark A. (October 8, 2008). "To boo or not to boo?". BBC. "“It just frustrates me when the crowd boo England, who is that going to help? It just heaps more pressure on the players and gives us even less of a chance of scoring”"
- Anthony Witrado (July 9, 2012). Home Run Derby 2012: Fans' treatment of Robinson Cano strengthens case for changes. Sporting News - MLB. "Kansas City fans mercilessly booed New York Yankees second baseman and American League captain Robinson Cano on Monday night, cheering with passion every time he made an out and even louder when he was shut out in the contest at Kauffman Stadium."
- Peter Botte (July 7, 2008). Derek Jeter hears the boos during Thursday's loss to Red Sox. NY daily News.
- Peter Botte (March 11, 2012). Boo birds serenade Knicks in loss to 76ers. NY daily News. "Carmelo Anthony heard noticeable boos during pre-game introductions, but Mike D’Antoni and the rest of the Knicks deservedly felt the crowd’s wrath, too, for the ugliness that transpired thereafter Sunday afternoon at the Garden. The negativity got so bad late in the Knicks’ fifth straight loss, portions of the crowd chanted “Fire D’Antoni” — the natural progression from the boos that poured down from the stands through much of the second half."