A detective is an investigator, either a member of a police agency or a private person. They may be known as private investigators (P.I.s or "Private I's", hence the play-on-words, "Private Eyes"). Informally, and primarily in fiction, a detective is any licensed or unlicensed person who solves crimes, including historical crimes, or looks into records.

Selection and training

In most American police departments, a candidate for detective must first have served as a uniformed officer for a period of one to five years. Detective is often an appointed position, rather than a position achieved by passing a written test. Prospective U.K. police detectives must have completed at least two years as a uniformed officer before applying to join the Criminal Investigation Department. UK Police must also pass the National Investigators' Examination in order to progress on to subsequent stages of the Initial Crime Investigators Development Programme in order to qualify as a Detective.

In many other European police systems, most detectives are university graduates who join directly from civilian life without first serving as uniformed officers. Some peopleWho: date=July 2008 argue that detectives do a completely different job and therefore require completely different training, qualifications, qualities and abilities than uniformed officers. The opposingWho: date=July 2008 argument is that without previous service as a uniformed patrol officer, a detective cannot have a great enough command of standard police procedures and problems and will find it difficult to work with uniformed colleagues.

Additionally, in some U.S. police departments, policies exist that limit the term that an officer may serve continuously as a detective, and mandate that detectives must regularly return to patrol duties for a minimum period of time. This is based upon a perceptionFact: date=July 2008 that the most important and essential police work is accomplished on patrol, and that the skills, experience and familiarity with their beats that patrol officers maintain are essential for detectives to maintain as well. Investigations, by contrast, often take weeks or months to complete, during which time detectives may spend much of their time away from the streets. In this thinking, rotating officers also promotes cross-training in a wider variety of skills, producing both better detectives and uniformed officers. Such policies also serve to prevent "cliques" within detective bureaus that can contribute to corruption or other unethical behavior.Fact: date=July 2008

Detectives obtain their position by competitive examination covering such subjects as principles, practices and procedures of investigation; interviewing and interrogation; criminal law and procedures; applicable law governing arrests, search and seizures, warrants and evidence; police department records and reports; principles, practices and objectives of courtroom testimony; and police department methods and procedures.