Polygraph is a general term that refers to the use of autonomic physiological measures to make assessments about a person's credibility. Polygraph techniques find wide application in the criminal justice and national security systems of many countries, and their use is growing worldwide. There are two major families of polygraph techniques. Knowledge approaches look for responses that indicate knowledge possessed by a person attempting deception. Deception approaches assess credibility by examining a person's response to accusatory questions that directly address the issues under investigation. Both approaches have strengths and weaknesses, and both are the subject of controversy in the scientific literature. This entry describes the approaches, their strengths and weaknesses, their application in practice, and the controversy concerning them.