A private investigator (often abbreviated to PI and informally called a private eye), a private detective or inquiry agent, is a person who can be hired by individuals or groups to undertake investigatory law services. Private detectives/investigators often work for attorneys in civil cases. A handful of very skilled private detectives/investigators work with defense attorneys on capital punishment and criminal defense cases. Many work for insurance companies to investigate suspicious claims. Before the advent of no-fault divorce, many private investigators were hired to search out evidence of adultery or other conduct within marriage to establish grounds for a divorce. Despite the lack of legal necessity for such evidence in many jurisdictions, according to press reports collecting evidence of adultery or other "bad behaviour" by spouses and partners is still one of the most profitable activities investigators undertake, as the stakes being fought over now are child custody, alimony, or marital property disputes.
Private investigators can also be used to perform due diligence for an investor who may be considering investing money with an investment group, fund manager or other high-risk business or investment venture. This could serve to help the prospective investor avoid being the victim of a fraud or Ponzi scheme. By hiring a licensed and experienced investigator, they could unearth information that the investment is risky and or that the investor has suspicious red flags in his or her background. This is called investigative due diligence, and is becoming much more prevalent in the 21st century with the public reports of large-scale Ponzi schemes and fraudulent investment vehicles such as Madoff, Stanford, Petters, Rothstein and the hundreds of others reported by the SEC and other law-enforcement agencies.
Many jurisdictions require PIs to be licensed, and may or may not carry firearms depending on local laws. Some are former police officers, some are former law enforcement agents, some are former spies and some are former military, some used to work in a private military company, some are former bodyguards and security guards, although many are not. While PIs may investigate criminal matters, most do not have police powers, and as such they have only the powers of citizen's arrest and detainment that any other citizen has. They are expected to keep detailed notes and to be prepared to testify in court regarding any of their observations on behalf of their clients. Great care is required to remain within the scope of the law, otherwise the investigator may face criminal charges. Irregular hours may also be required when performing surveillance work.
Private Investigators in Australia must be licensed by the licensing authority relevant to the State they are located in. This applies to all States except the Australian Capital Territory. Companies offering investigation services must also hold a business licence as well as all operators holding an individual licence. Generally the licences are administered and regulated by the State Police, however in some states this can also be managed by other government agencies. 
In 2001, the government passed the licensing of private investigators and private investigation firms in UK and Wales over to SIA (Security Industry Authority act), who acted as the regulatory body from then on. However, due to the cutbacks of this SIA, licensing of private investigators in the UK was halted indefinitely. As of the moment there are no governmental backed authorities in the UK to license private investigators.
PIs also engage in a large variety of work that is not usually associated with the industry in the mind of the public. For example, many PIs are involved in process serving, the personal delivery of summons, subpoenas and other legal documents to parties in a legal case. The tracing of absconding debtors can also form a large part of a PI's work load. Many agencies specialize in a particular field of expertise. For example, some PI agencies deal only in tracing. Others may specialize in technical surveillance counter-measures (TSCM), sometimes called electronic counter measures (ECM), which is the locating and dealing with unwanted forms of electronic surveillance (for example, a bugged boardroom for industrial espionage purposes). Other PIs, also known as Corporate Investigators, specialise in corporate matters, including anti-fraud work, the protection of intellectual property and trade secrets, anti-piracy, copyright infringement investigations, due diligence investigations and computer forensics work. Some PIs act as professional witnesses where they observe situations with a view to reporting the actions or lack of them to a court or to gather evidence in anti-social behavior..
Undercover investigator 
An undercover investigator, undercover detective, or undercover agent is a person who conducts investigations of suspected or confirmed criminal activity while impersonating a disinterested third party. Undercover investigators often infiltrate a suspected insurgent group, posing as a person interested in purchasing illegal goods or services with the ultimate aim of obtaining information about their assigned target.
Many undercover investigators carry hidden cameras and recorders strapped to their bodies to help them document their investigations. The period of the investigation could last for several months or, in some extreme cases, years. Due to the dangerous nature of the job, their real identities are kept secret throughout their active careers.
Modern private investigators 
Increasingly, modern PIs prefer to be known as "professional investigators" or Licensed Private Investigators (LPI's) rather than "private investigators" or "private detectives". This is a response to the image that is sometimes attributed to the profession and an effort to establish and demonstrate the industry to be a proper and respectable profession. However, in 2009 a Toronto Star journalist obtained a private investigator's license in Ontario with no training, and reported that other Ontarians had done the same.
In 1833 Eugène François Vidocq, a French soldier, criminal and privateer, founded the first known private detective agency, "Le Bureau des Renseignements Universels pour le commerce et l'Industrie" ("The Office of Universal Information For Commerce and Industry") and hired ex-convicts. Official law enforcement tried many times to shut it down. In 1842 police arrested him in suspicion of unlawful imprisonment and taking money on false pretences after he had solved an embezzlement case. Vidocq later suspected that it had been a set-up. He was sentenced for five years with a 3,000-franc fine but the Court of Appeals released him. Vidocq is credited with having introduced record-keeping, criminology and ballistics to criminal investigation. He made the first plaster casts of shoe impressions. He created indelible ink and unalterable bond paper with his printing company. His form of anthropometrics is still partially used by French police. He is also credited for philanthropic pursuits – he claimed he never informed on anyone who had stolen for real need.
After Vidocq, the industry was born. Much of what private investigators did in the early days was to act as the police in matters that their clients felt the police were not equipped for or willing to do. A larger role for this new private investigative industry was to assist companies in labour disputes. Some early private investigators provided armed guards to act as a private militia.
In the United Kingdom, Charles Frederick Field set up an enquiry office upon his retirement from the Metropolitan Police in 1852. Field became a friend of Charles Dickens and the latter wrote articles about him. In 1862 one of his employees, the Hungarian, Ignatius Paul Pollaky, left him and set up a rival agency. Although little remembered today, Pollaky's fame at the time was such that he was mentioned in various books of the 1870s and immortalized as "Paddington" Pollaky for his "keen penetration" in the 1881 comic opera, Patience.
In the United States, the Pinkerton National Detective Agency was a private detective agency established in 1850 by Allan Pinkerton. Pinkerton had become famous when he foiled a plot to assassinate then President-Elect Abraham Lincoln. Pinkerton's agents performed services which ranged from undercover investigations and detection of crimes to plant protection and armed security. It is sometimes claimed, probably with exaggeration, that at the height of its existence the Pinkerton National Detective Agency employed more agents than the United States Army. Allan Pinkerton hired Kate Warne in 1856 as a private detective, making her the first female private detective in America.
During the union unrest in the US in the late 19th century, companies sometimes hired operatives and armed guards from the Pinkertons. In the aftermath of the Homestead Riot, several states passed so-called "anti-Pinkerton" laws restricting the importation of private security guards during union strikes. The federal Anti-Pinkerton Act of 1893 continues to prohibit an "individual employed by the Pinkerton Detective Agency, or similar organization" from being employed by "the Government of the United States or the government of the District of Columbia."
Pinkerton agents were also hired to track western outlaws Jesse James, the Reno brothers, and the Wild Bunch, including Butch Cassidy and the Sundance Kid. The Pinkerton agency's logo, an eye embellished with the words "We Never Sleep," inspired the term "private eye."
It was not until the prosperity of the 1920s that the private investigator became a person accessible to the average American. With the wealth of the 1920s and the expanding of the middle class came the need of middle America for private investigators.
Since then, the private detective industry has grown with the changing needs of the public. Social issues like infidelity and unionization have impacted the industry and created new types of work, as has the need for insurance and, with it, insurance fraud, criminal defence investigations and the invention of low-cost listening devices.
For state licensing, some states require a clean criminal record at the application entry point. However, all states have a board of directors who will review an applicants appeal to determine whether the board can approve the application based on the elapsed amount of time since the last offence was recorded. In most situations, the board of appeal will approve an application based on good conduct within the last five to ten years. This has combined with modern business practices that have ensured that most investigators are now professional in outlook, rather than seeing the PI world as a second career opportunity for retired policemen.
Notable private investigators 
- AnnaMaria Cardinalli
- Charles Frederick Field
- Joe Gores
- Leonard Padilla
- Vincent Parco
- Anthony Pellicano
- Daniel Ribacoff
See also 
|Wikimedia Commons has media related to: Private investigators|
- "Private Detectives and Investigators". United States Department of Labor, Occupational Outlook Handbook, 2010-2011 Edition. 2010.
- Popplewell, Brett (2009-09-18). "$80 and I'm a security guard". The Star (Toronto). Retrieved 2010-05-22.
- Historique des détectives et enquêteurs privés et grandes dates de la profession – "Le Bureau des Renseignements Universels pour le commerce et l'Industrie”
- "Kate Warne America's First female Private-Eye". Pimall.com. Retrieved 2013-02-14.
- 5 U.S. Code 3108; Public Law 89-554, 80 Stat. 416 (1966); ch. 208 (5th par. under "Public Buildings"), 27 Stat. 591 (1893). The U.S. Court of Appeals for the Fifth Circuit, in U.S. ex rel. Weinberger v. Equifax, 557 F.2d 456 (5th Cir. 1977), cert. denied, 434 U.S. 1035 (1978), held that "The purpose of the Act and the legislative history reveal that an organization was 'similar' to the Pinkerton Detective Agency only if it offered for hire mercenary, quasi-military forces as strikebreakers and armed guards. It had the secondary effect of deterring any other organization from providing such services lest it be branded a 'similar organization.'" 557 F.2d at 462; see also "GAO Decision B-298370; B-298490, Brian X. Scott (Aug. 18, 2006).".
- "Pinkertons". Tom-horn.com. Retrieved 2013-02-14.