A private investigator, private detective, PI, or private eye, is a person who can be hired by individuals, groups, or businesses 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.