Private investigators in the United States may or may not be licensed or registered by a government licensing authority or state police of the state where they are located. Licensing varies from state to state and can range from. In many states, companies offering investigation services must hold an agency license, and all of their investigators or detectives must hold individual licenses or registrations, furthermore, certain states such as Washington have separate classes of licensing for roles such as trainers of private investigators. A few reciprocity agreements allow a detective working in one state to continue work in another for a limited time without getting a separate license, but not all states participate in these agreements.

A private investigator (often abbreviated to PI and informally called a private eye), a private detective, or inquiry agent is a person who can be hired by individuals, groups or NGOs to undertake investigatory law services. Private investigators often work for attorneys in civil and criminal cases.

An undercover investigator, undercover detective, or undercover agent is a person who conducts investigations of suspected or confirmed criminal activity while impersonating a disinterested third party. Undercover investigators often infiltrate a suspected insurgent group, posing as a person interested in purchasing illegal goods or services with the ultimate aim of obtaining information about their assigned target.

Many undercover investigators carry hidden cameras and recorders strapped to their bodies to help them document their investigations. The period of the investigation could last for several months, or in some extreme cases, years. Due to the dangerous nature of the job, their real identities are kept secret throughout their active careers. Economic investigations, business intelligence and information on competitors, security advice, special security services information, criminal investigation, investigations background, and profile polygraph tests are all typical examples of such a role.

Certain types of undercover investigators, depending on their employer, will investigate allegations of abuse of workman's compensation. Those claiming to be injured are often investigated and recorded with a hidden camera/recorder. This is then presented in court or to the client who paid for the investigation.