According to the Bureau of Labor Statistics, private detectives and private investigators are often hired to help individuals, businesses, and attorneys. Their responsibilities often include finding and analyzing information and connecting clues to uncover facts pertaining to their clients' legal, financial, or personal matters. This profession has been around since the early 19th century.

The first private investigator in history created his private investigation firm in 1833. Eugène François Vidocq was a French criminal and privateer. He made many advances in the field, and is often credited with introducing record-keeping, criminology, and ballistics to the process of criminal investigation. In addition, he was the first to create plaster casts of shoe impressions - techniques that are still widely used today. Vidocq's private investigation firm sparked a new industry for retired police and military officers, as well as the average citizen who wishes to become a private investigator.

One of the first detective agencies in the U.S. was the Pinkerton National Detective Agency - established by Allan Pinkerton in 1850. Perhaps most famous for foiling the assassination plot on President Lincoln, Pinkerton's private investigators often performed undercover investigations and armed security. In the late 19th century, during a period of union unrest in the US, many Pinkerton agents were hired as armed guards and operatives to keep the strikers and unionists away from the factories. Pinkerton agents were also used to track famous western outlaws, including Butch Cassidy and the Sundance Kit, the Wilt Bunch, and Jesse James. Their logo was an eye and the words "We Never Sleep" - this inspired the widely known and still used term of "Private Eye."

That first private investigation agency has become an entire industry that assists clients with child custody cases, marital infidelity investigations, child abuse investigations, executive protection, background investigations, and computer forensics. The rise of social issues in today's world, such as infidelity and unionization, has created new types of work for private investigation agencies. Insurance fraud had also become more common, and therefore private investigators have had to start investigating fraud. In addition to these services, many private investigators also provide services that aren't typically associated with the profession, including the personal delivery of summons, tracking of debtors, and technical surveillance counter-measures.

Many individuals who choose to go into the field of private investigation are ex-police officers or ex-military, former bodyguards or security guards. Many, however, are simply average citizens with no military or police experience. Most private investigators are required to be licensed, and they must keep detailed notes and be ready and willing to testify in court.

Although a private investigator's job is to investigate, there are some things that they are not allowed to do. The laws concerning the abilities of private investigators vary from state to state and country to country. Because they are still private citizens, private investigators are not permitted by law to wiretap phones without consent, trespass on private property, tamper with mail, make an arrest, or impersonate law enforcement.