A private investigator, private detective, PI, or private eye, is a person who undertakes investigations, usually for a private citizen or some other entity not involved with a government or police organization. They 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.

Many jurisdictions require PIs to be licensed, and they may or may not carry firearms depending on local laws. Some are ex-police officers. 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 (e.g., being forbidden to trespass on private property or break into homes) is also required, on pain of losing their licenses as well as facing criminal charges. Irregular hours may also be required when performing surveillance work (e.g., outside someone's house during the early hours of the morning).

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).

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.