There are two primary types of detectives: police detectives and private detectives. Both require a great deal of training, and the ability to pass certain licensing exams which are used to confirm that the candidate is qualified and morally fit for the job. Many private detectives start out as police detectives, eventually retiring from the police force and taking on private work, so most people become a detective by joining the police force, whether they want to work privately or with the police in the long term.

In order to join the police force to become a detective, the candidate will need a degree in criminal justice, and he or she will be required to pass background checks, interviews, and drug tests. If the candidate is accepted, it will be necessary to attend to the police academy before the candidate can serve as a police officer. After six months to two years, the officer can apply to work as a detective.

Police detectives are selected on the basis of experience, performance, and skills. Someone who consistently works hard and thoroughly as a police officer will be more likely to be chosen, as will someone who can clear cases efficiently and quickly. A police force may ask a candidate to gain more experience and reapply, which can be frustrating, but should be viewed as an opportunity to rack up more experience to support an application to become a detective. Once promoted to detective, a police officer can work his or her way up the ranks.

After someone has become a detective in the police and worked for a number of years, it is usually very easy to obtain a license to work as a private detective after retirement. People who do not qualify in the police will probably need to fulfill certain requirements in order to obtain a license from the state or government, such as education, ethics classes, and so forth. Private detectives must also pass background checks, which include interviews with friends and former coworkers, and other measures which are meant to test moral fitness. These requirements vary by region, and candidates should ask law enforcement agencies about the specific rules where they wish to work.

There is a third way to become a detective, for people who aren't interested in either of these options. Employees of certain government agencies have jobs which involve a great deal of detective work, and while these individuals may not be officially referred to as detectives, they do much of the same work that police detectives and private investigators do. It is also possible to become a private investigator or police detective after having served in a government agency which handles law enforcement issues.