Former law enforcement officers, military investigators, and government agents often become private investigators as a second career because they are frequently able to retire after 20 years of service. Others enter from such diverse fields as finance, accounting, commercial credit, investigative reporting, insurance, and law. These people often can apply their prior work experience in a related investigative specialty. A few enter the occupation directly after graduation from college, generally with associate or bachelor of criminal justice or police science degrees.

The majority of the States and the District of Colombia require private investigators to be licensed. Licensing requirements vary widely, but convicted felons cannot receive a license in most States and a growing number of States are enacting mandatory training programs for both private detectives and investigators. Some States have few requirements, and 6 States: Alabama, Alaska, Colorado, Idaho, Mississippi, and South Dakota, have no statewide licensing requirements while others have stringent regulations. For example, the Bureau of Security and Investigative Services of the California Department of Consumer Affairs requires private investigators to be 18 years of age or older; have a combination of education in police science, criminal law, or justice, and experience equaling 3 years (6,000 hours) of investigative experience; pass an evaluation by the Federal Department of Justice(DOJ) and a criminal history background check; and receive a qualifying score on a 2-hour written examination covering laws and regulations. There are additional requirements for a firearms permit.