Detective, a person employed to make investigations. A detective on a police force usually belongs to a plain-clothes division and is involved in crime investigation. On large police forces, detectives usually specialize, investigating crimes in specific fields, such as homicide, narcotics, vice, arson, or burglary. The federal government has its own detective agencies, the most important being the Federal Bureau of Investigation, the Secret Service, and the Postal Inspection Service.

Railways, hotels, large stores, and business corporations employ private detectives to prevent theft and damage to property and to ensure the safety of their customers. Other private detectives gather evidence for divorce cases, investigate persons' backgrounds for employers, and check on insurance claims. There are many private detective agencies, such as Pinkerton's Inc. and Burns International Security Services, Inc. Of the hundreds of fictional detectives, the most famous is probably Sir Arthur Conan Doyle's Sherlock Holmes.