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 investigators often work for attorneys in civil and criminal cases.

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"^[1] ("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 to five years and fined 3,000-francs, 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.^[2]

After Vidocq, the industry was born. Much of what private investigators did in the early days was to act as the police in matters for which their clients felt the police were not equipped or willing to do. A larger role for this new private investigative industry was to assist companies in labor disputes. Some early private investigators provided armed guards to act as a private militia.^[2]

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, Allan Pinkerton established the Pinkerton National Detective Agency – a private detective agency – in 1850. Pinkerton became famous when he foiled a plot to assassinate then President-elect Abraham Lincoln in 1861. 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.^[2] Allan Pinkerton hired Kate Warne in 1856 as a private detective, making her the first female private detective in America.^[3]

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 of 1892, 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."^[2][4]

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.

Many private detectives/investigators with special academic and practical experience also work with defense attorneys on capital punishment and other criminal defense cases. Many others are insurance investigators who investigate suspicious claims. Before the advent of no-fault divorce, many private investigators sought 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 spouses' and partners' adultery or other "bad behaviour" is still one of their most profitable undertakings, as the stakes being fought over now are child custody, alimony, or marital property disputes.^[2]

Private investigators can also perform due diligence for an investor considering investing with an investment group, fund manager, or other high-risk business or investment venture. This could help the prospective investor avoid being the victim of a fraud or Ponzi scheme. A licensed and experienced investigator could reveal the investment is risky and/or the investor has a suspicious background. This is called investigative due diligence, and is becoming 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 Securities and Exchange Commission along with other law enforcement agencies.

Private investigators also engage in a variety of work not often associated with the industry in the mind of the public. For example, many 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. A handful of firms specialize in technical surveillance counter-measures, sometimes called electronic counter measures, which is the locating and dealing with unwanted forms of electronic surveillance (for example, a bugged boardroom for industrial espionage purposes). This niche service is typically conducted by those with backgrounds in intelligence/counterintelligence, executive protection, and a small number from law enforcement entities whose duties included the covert installation of eavesdropping devices as a tool in organized crime, terrorism and narco-trafficking investigations. Other PIs, also known as corporate investigators, specialize in corporate matters, including antifraud work, loss prevention, internal investigations of employee misconduct (such as Equal Employment Opportunities violations and sexual harassment), the protection of intellectual property and trade secrets, antipiracy, copyright infringement investigations, due diligence investigations, malware and cyber criminal activity, 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 antisocial behavior.^[2]

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.^[5]

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.^[6] Economic investigations, business intelligence and information on competitors, security advice, special security services information, criminal investigation, investigations background, and profile polygraph tests are all typical examples of such a role.

Certain types of undercover investigators, depending on their employer, will investigate allegations of abuse of workman's compensation. Those claiming to be injured are often investigated and recorded with a hidden camera/recorder. This is then presented in court or to the client who paid for the investigation.

Many jurisdictions require PIs to be licensed. Depending on local laws, they may or may not carry a firearm, some are former law enforcement agents (including former police officers), some are former spies, some are former military, some used to work for a private military company, and some are former bodyguards and security guards. While PIs may investigate criminal matters, most do not have police authority, and as such, they are only limited to the powers of citizen's arrest and detention 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.^[2]

Private investigators in Australia must be licensed by the licensing authority relevant to the state where they are located. This applies to all states except the Australian Capital Territory^[7]. Companies offering investigation services must also hold a business licence and all their operatives must hold individual licences. Generally, the licences are administered and regulated by the state police; however, in some states, this can also be managed by other government agencies. ^[8]

To become registered in New South Wales requires a CAPI licence, which can be applied for through the NSW Police Force website.^[9] The Australian Capital Territory does not require PIs to be licensed, although they are still bound by legislation. PIs working in the ACT cannot enter the NSW area without a CAPI license, else they will be in breach of the law.^[10] In Queensland, a private investigator need to be licensed under the Queensland Government and apply for a private investigator licence^[11] by completing an application for a security provider licence. Applicant will need to have a criminal history check and submit fingerprint.

In 2001, the government passed the licensing of private investigators and private investigation firms in the UK over to the Security Industry Authority (SIA), which acted as the regulatory body from then on. However, due to the cutbacks of this agency, licensing of private investigators ^[12] in the UK was halted indefinitely. At present, no government-backed authorities in the UK license private investigators.

The SIA have announced that PIs in the UK were to become licensed for the first time from May 2015, but this is only the scheduled date for the issue to be discussed in parliament. In December 2014, Corporate Livewire produced an article written by a UK private investigator at BAR Investigations, addressing the issues surrounding private investigation in the UK.^[13][14]

Private investigators in the United States may or may not be licensed or registered by a government licensing authority or state police of the state where they are located. Licensing varies from state to state and can range from: a) no state license required; b) city or state business license required (such as in five states (Idaho, Alaska, Mississippi, South Dakota, and Wyoming); c) to needing several years of experience and licensing-related training classes and testing (as is the case with Virginia and California).^[15] In many states, companies offering investigation services must hold an agency license, and all of their investigators or detectives must hold individual licenses or registrations; furthermore, certain states such as Washington have separate classes of licensing for roles such as trainers of private investigators.^[16] A few reciprocity agreements allow a detective working in one state to continue work in another for a limited time without getting a separate license, but not all states participate in these agreements.^[17]

In 1877, Colorado became the first state in the union to institute licensing requirements for private investigators. Because of the vague definition of term 'private investigator', the law was declared unconstitutional in 1977, but reinstated on a voluntary basis in July 2012 and mandatory in June 2015.^[18][19]

Florida has 3 types of licenses - Class CC for private investigator intern, C for private investigator, and MA for manager of a private investigative agency. As Class C license requires at least 2 years of experience, most applicants start with Class CC, which allows them to work under a sponsorship of a licensed Class C investigator.^[20]

In 1893 a federal law was passed specifically barring the government from employing the Pinkerton Detective Agency or similar agency.

Private investigators in Canada are licensed at the provincial level by the appropriate body. For instance, in the province of Ontario, private investigators are licensed and regulated by the Ministry of Community Safety & Correctional Services (MCSCS).^[21] In the province of Alberta, private investigators are licensed and regulated by the Alberta Justice and Solicitor General.^[22] Similar licensing requirements apply in other provinces and territories of Canada. As per the Ontario text of the Private Security and Investigative Services Act of 2005 ^[24] , private investigators are forbidden from referring to themselves as detective or private detective. In order to become a licensed private investigator, you must be 18 years of age or older in Ontario (in other Provinces and territories of Canada the eligible age to work may be higher); have a clean criminal record or obtain a waiver; and submit a correctly completed application for a license. You are required to complete 50-hours of basic training with an accredited source such as a university, college, or through private agencies licensed to administer the course. Upon completion of basic training, individuals are required to write and pass the basic test to obtain a private investigator's license.

There are a number of global networks which helps Private Investigators to stay connected, share resources and carry out work across borders. Some of these are World Association of Detectives and The Association of British Investigators. These organisations promote ethical practices in the profession of Private Investigator or security service throughout the world.

The PI genre in fiction dates to Edgar Allan Poe, who created the character C. Auguste Dupin in the 1840s. Dupin, an amateur crime-solver residing in Paris, appeared in three Poe stories.

Where the characters below do not meet the strict criteria of a private investigator (i.e. available for hire) it is noted in brackets.