A private investigator or private detective, is a professional who undertakes investigations independently. Such investigations can be requested by private citizens who need to get information difficult to gather by themselves such as evidence of illegal or bad behavior by relatives or partners, for child custody and protection cases, to find missing people or even to do premarital screenings.

Private investigators are also great support for attorneys, who frequently rely on detectives to obtain vital information for civil and liability cases or to gather facts to defend people who are being charged with criminal charges. Also frequent customers for these qualified experts are insurance companies, which many times use private investigators when in need to look into suspicious claims, or companies looking out for debtors, to protect their property and other assets, or to investigate fraudulent workers' compensation claims. The possibilities are endless.

The work of a private investigator is often done through interviews, investigation, surveillance, and research, including the review of public documents and records. Because of the nature of this work, their activities frequently take place at unconventional hours and places. Some times the investigator will take video or photographs to document the facts and to report them to its clients.

A private investigator should not take on its own opinions or suppositions. He should only gather, organize and report his findings in order to help put together an unbiased explanation. It is the client whom, from its own perspective, should evaluate the nature of the findings and draw its own conclusions. It is not rare for a private investigator to testify in court, and put together evidence and reports to be used in trials.