A private investigator (often abbreviated to PI), private detective or (informally) private eye 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. 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.

Many jurisdictions require PIs to be licensed, and they may or may not carry firearms depending on local laws. Some are ex-police officers, some are former federal agents, some are ex-spies and some are ex-military, some used to work in a private military company, some are former bodyguards and security guards, although many are not. Most of them do not arrest criminals or put them in custody. 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.

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.

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 licence in Ontario with no training, and reported that other Ontarians had done the same.

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