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A polygraph, commonly known as a lie detector, is a device that measures and records several physiological variables such as blood pressure, pulse, respiration, and skin conductivity while a person is asked a series of questions. The theory behind the polygraph is that deceptive answers will produce distinct physiological responses that can be measured by the instrument.

The polygraph examination process typically involves a trained examiner who attaches sensors to the subject. The subject is then asked a series of questions, which may include control questions designed to elicit a known physiological response, and relevant questions pertaining to the matter under investigation. The examiner analyzes the recorded physiological data, looking for significant deviations in response to the relevant questions compared to the control questions.

Polygraph results are not admissible as evidence in most legal systems due to concerns about their scientific validity and reliability. The accuracy of polygraph tests is a subject of ongoing debate. Factors such as the skill of the examiner, the psychological state of the subject, and the specific countermeasures employed can all influence the outcome. Despite these limitations, polygraph examinations are sometimes used in pre-employment screening for certain government positions and in criminal investigations, often on a voluntary basis.