|This article or section deals primarily with the United States and does not represent a worldwide view of the subject.|
Please improve this article or discuss the issue on the talk page.
A private investigator, private detective, PI, or private eye, is a person who can be hired by individuals or groups to undertake investigations.
Private investigators often work for attorneys in civil cases or on behalf of a defense attorney. 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 illegal 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 behavior" 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.
Many jurisdictions require PIs to be licensed, and they may or may not carry firearms depending on local laws. Some are ex-police officers, although many are not. 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. Taking great care to remain within the law in the scope is also required, as this may lead to the individual facing criminal charges. Irregular hours may also be required when performing surveillance work.
PIs also undertake 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 countermeasures, or TSCM, 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.
Increasingly, modern PIs prefer to be known as "professional investigators" rather than "private investigators" or "private detectives". This is a response to the seedy image that is sometimes attributed to the profession and an effort to establish and demonstrate the industry to be a proper and respectable profession.
In some countries throughout the world, private investigations are illegal. In other countries, private investigators thrive, including: United States, the Netherlands, Mexico, Canada, Turkey, United Kingdom, France, Spain, South Africa, Australia and Japan. In South Africa, private investigators are in very high demand due to poor police work and high crime rates. Other countries throughout the world have private investigators, but many of their duties are restricted. In South Korea, for example, surveillance is allowed only in insurance fraud situations. Some countries in the world require licensing of private detectives, but most do not. 
Private detectives and investigators often work irregular hours because of the need to conduct surveillance and contact people who are not available during normal working hours. Early morning, evening, weekend, and holiday work is common.
Many detectives and investigators spend time away from their offices conducting interviews or doing surveillance, but some work in their office most of the day conducting computer searches and making telephone calls. Those who have their own agencies and employ other investigators may work primarily in an office and have normal business hours.
When the private investigator is working on a case away from the office, the environment might range from plush boardrooms to seedy bars. Store and hotel detectives work in the businesses that they protect. Investigators generally work alone, but they sometimes work with others during surveillance or when following a subject in order to avoid detection.
Some of the work involves confrontation, so the job can be stressful and dangerous. Some situations call for the investigator to be armed, such as certain bodyguard assignments for corporate or celebrity clients. Detectives and investigators who carry handguns must be licensed by the appropriate authority. In most cases, however, a weapon is not necessary, because the purpose of the work is gathering information and not law enforcement or criminal apprehension.
There are no formal education requirements for most private detective and investigator jobs, although many private detectives have college degrees or have taken legal or criminal investigation courses. Private detectives and investigators typically have previous experience in other occupations. Some work initially for insurance or collections companies, in the private security industry, or as paralegals. Many investigators enter the field after serving in law enforcement, the military, government auditing and investigative positions, or federal intelligence jobs.
Former law enforcement officers, military investigators, and government agents, who are frequently able to retire after 20 or 25 years of service, often become private detectives or investigators in a second career. Others enter from such diverse fields as finance, accounting, commercial credit, investigative reporting, insurance, and law. These individuals often can apply their prior work experience in a related investigative specialty. A few enter the occupation directly after graduation from college, generally with associate's or bachelor's degrees in criminal justice, police science or with a private investigation diploma.
The majority of states in the United States and the District of Columbia require private detectives and investigators to be licensed. Licensing requirements vary, however. Seven states—Alabama, Alaska, Colorado, Idaho, Mississippi, Missouri, and South Dakota—have no statewide licensing requirements, some states have very few requirements, and many other states have stringent regulations. A growing number of states are enacting mandatory training programs for private detectives and investigators. For example, the Bureau of Security and Investigative Services of the California Department of Consumer Affairs requires private investigators to be 18 years of age or older, have a combination of education in police science, criminal law, or justice and experience equalling 3 years (6,000 hours) of investigative experience, pass a criminal history background check by the California Department of Justice and the FBI (in most states, convicted felons cannot be issued a license), and receive a qualifying score on a two-hour written examination covering laws and regulations. There are additional requirements for a firearms permit.
For private detective and investigative jobs, most employers look for individuals with ingenuity, persistence, and assertiveness. A candidate must not be afraid of confrontation, should communicate well, and should be able to think on his or her feet. Good interviewing and interrogation skills also are important and usually are acquired in earlier careers in law enforcement or other fields. Because the courts often are the ultimate judge of a properly conducted investigation, the investigator must be able to present the facts in a manner that a jury will believe.
Training in subjects such as criminal justice and police science can be helpful to aspiring private detectives and investigators. Most corporate investigators must have a bachelor's degree, preferably in a business-related field. Some corporate investigators have a master's degree in business administration or a law degree, while others are CPAs. Corporate investigators hired by large companies may receive formal training from their employers on business practices, management structure, and various finance-related topics. The screening process for potential employees typically includes a background check for a criminal history.
Some investigators receive certification from a professional organization to demonstrate competency in a field. For example, the National Association of Legal Investigators (NALI) confers the Certified Legal Investigator designation to licensed investigators who devote a majority of their practice to negligence or criminal defense investigations. To receive the designation, applicants must satisfy experience, educational, and continuing-training requirements and must pass written and oral exams administered by the NALI.
Most private-detective agencies are small, with little room for advancement. Usually, there are no defined ranks or steps, so advancement takes the form of increases in salary and assignment status. Many detectives and investigators work for detective agencies at the beginning of their careers and, after a few years, start their own firms. Corporate and legal investigators may rise to supervisor or manager of the security or investigations department.
In 1833 Eugène François Vidocq, a French soldier, criminal and privateer, founded the first known private detective agency, "Le bureau des renseignments" (Office of Intelligence) 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 embezzling 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 to was to assist companies in labor disputes. Some early private investigators provided armed guards to act as a private militia.
In the U.S., 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. 
During the labor unrest of the late 19th century, companies sometimes hired operatives and armed guards from the Pinkertons and similar agencies to keep strikers and suspected unionists out of their factories. The most famous example of this was the Homestead Strike of 1892, when industrialist Henry Clay Frick hired a large contingent of Pinkerton men to regain possession of Andrew Carnegie's steel mill during a lock-out at Homestead, Pennsylvania. Gunfire erupted between the strikers and the Pinkertons, resulting in multiple casualties and deaths on both sides. Several days later a radical anarchist, Alexander Berkman, attempted to assassinate Frick. In the aftermath of the Homestead Riot, several states passed so-called "anti-Pinkerton" laws restricting the importation of private security guards during labor 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 defense investigations and the invention of low-cost listening devices. In a number of countries, a licensing process has been introduced that has put criteria in place that investigators have to meet: in most cases, a clean criminal record. 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.
The PI genre in fiction dates back to Edgar Allan Poe who created the character C. Auguste Dupin, who lived in Paris. The genre graduated onto cinema and television screens and is still a regular theme on stage, screen and book.