A polygraph, also known as lie detector, is a device used to detect lies. The term "polygraph" literally means "writing much" (from Greek polys "much" + graphos "writing"). Polygraph measures and records changes in cardiovascular patterns, respiratory patterns and in sweat gland activity during an interrogation.

The original polygraph was invented by forensic psychiatrist John Larson in 1921 in California.

He built a machine that simultaneously measured pulse rate, blood pressure, and respiratory changes while a series of questions was being asked. Later he developed an interviewing technique where relevant (Did you steal the money?) and irrelevant (Are you 30 years old?) questions alternate.

While not one scientific study that could prove the validity of the polygraph test has been published so far, American Polygraph Association states that polygraph is between 85% and 95% accurate. The opponents, however, believe that the problem with polygraph testing lies in the interpretation of the test results. The study of six polygraph interpreters in 1981 showed that even the most experienced of them had an error rate of 18 percent. Therefore, some regard polygraph tests rather as tools of intimidation than instruments of truth.

Polygraph tests are common in police investigations in the US, though no suspect can be forced to undergo the test.

In most European jurisdictions lie detectors are not generally used by police, and are not considered reliable evidence in courts.