A private investigator, or PI, is a person who undertakes investigations. They often work for attorneys in civil cases or on behalf of a defense attorney. Many work for insurance companies to resolve 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 any more, 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.

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 licences 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).