A polygraph test, also known as a lie detector test, is a machine or procedure that measures and records several physiological indicators such as blood pressure, pulse, respiration, and skin conductivity while a person is asked questions. The purpose of the test is usually to prove whether or not a person committed a crime, but it cannot actually test for honesty. Instead, it relies on the polygraph operators analysis of the tested persons responses, which can be inaccurate. Polygraphs are usually not admissible as evidence in United States courts.

The examiner typically begins polygraph test sessions with a pre-test interview to gain some preliminary information which will later be used to develop diagnostic questions. Then the tester will explain how the polygraph is supposed to work, emphasizing that it can detect lies and that it is important to answer truthfully. Then a "stim test" is often conducted: the subject is asked to deliberately lie and then the tester reports that he was able to detect this lie.

There are two types of polygraph examinations used in security clearance and employment screenings: counterintelligence and lifestyle. Counterintelligence covers questions of espionage, sabotage, and terrorist activities and is designed to root out contact with a foreign national or the compromise of classified information. Lifestyle deals more with the personal questions you answered on your SF-86, such as illegal activity, drug use, or falsification of the security clearance forms.

Polygraph examination results are not court admissible because they are considered fundamentally unreliable by the court and there is no consensus among scientists about their accuracy.