Police detectives in the United Kingdom are part of a special division known as the Criminal Investigation Department or CID. They are known informally as plain-clothes officers, in contrast to their uniformed colleagues. In the summer of 2009, reports said there was a national shortfall of 5,000 CID officers.

Choose your region. The national police is divided into regional police forces, often known as constabularies. Although subject to national government control, the forces largely act independently and recruit new members directly. There are also several non-geographic forces specializing in transport, organized crime and the military among other topics.

Before becoming a detective, candidates must spend two years as a uniformed officer. They will need to pass an assessment, medical examinations and background and security tests before being accepted for training. Once accepted, candidates will undergo an extensive training period, which includes classroom and on-the-job training, including working in a variety of departments across a force.

Find a vacancy. Police officers cannot automatically join their force's CID and become a detective. They must wait for a vacancy and apply for the role. It is likely they will face competition from other officers. The criteria on which applicants are judged will depend on the specific force, but will be specific to the role of a detective. This means that somebody who performs very well in the role of a uniformed officer may not necessarily be the best candidate for a detective. To give an exaggerated example, a particularly tall and burly man may be very effective in a uniformed role, for example in deterring troublemakers, but might be less able to carry out an undercover role inconspicuously. There are no specific physical requirements or barriers though, and each case is judged on its merits.