This is also called a lie detector test, but this term is a misnomer. The theory behind polygraph tests is that ability of subject is more likely to be concerned with lying about the relevant facts about the crime, which in turn produces a hyper arousal state which is picked up by a person trained in reading polygraph results. Measurement of hyper arousal state is based on a number of parameters such as heart rate, blood pressure, respiratory rate, skin conductance and electromyography. The brief idea underpinning the use of the polygraph is that deceptive answers tend to produce physiological responses that can be differentiated from those associated with non-deceptive answers. There are, however, no specific physiological reactions associated with lying, making it difficult to identify factors that separate liars from truth tellers. Polygraph examiners also prefer to use their own individual scoring methods, as opposed to computerized techniques, as they may more easily defend their own evaluations.
Mode of operation
Polygraph testing combines interrogation with physiological measurements obtained through the polygraph, or polygraph instrument, a piece of equipment that records physiological phenomena-typically, respiration, heart rate, blood pressure and electro-dermal response. In this sense, a polygraph examination includes a series of questions (yes/no) to which the examinee responds while connected to sensors that transmit data to the instrument, which uses analog or digital technology to record the data. The original analog instruments recorded the data in the form of graphs generated by several pans on a moving sheet of paper. Hence the record of physiological responses during the testing is known as the polygraph chart. In the modern are with the development of electronic devices, a variety of other physiological responses are recorded and are used to make inferences about deceptiveness. These range from brain scans to analysis of voice tremors. 
In India, the results of polygraph examination are not accepted as a sole evidence in court. The main reason is that the scientific community feels that the test is far from infallibility. Despite this fact, polygraph is extensively used in criminal interrogations in India. There is no provision in the Criminal Procedure Code, Evidence Act and Constitution to accept the results of polygraph test. However, police in India have been using it as an aid to interrogation.
Techniques Used 
Relevant/Irrelevant Question Technique (RQT)
In this technique, both relevant and irrelevant questions, pertaining to the case in hand are asked. The difference in the physiological response to relevant and irrelevant questions indicates whether the person is lying or not.
Peak of Tension Technique (POT)
In this technique, only one pertinent but emotionally charged question, after several controlled questions, of irrelevant nature, is asked. The irrelevant questions build the tension, which is released after the relevant question has been asked. The technique is dependent for its usefulness on the fact that a guilty person may possess knowledge which no innocent person could possess. And question regarding this knowledge would produce emotional reactions in a guilty person, while innocent person would show no reaction to the question.
Controlled Question Technique (CQT or RCQT)
In this technique, controlled questions are asked. These questions relate to the associative evidence already known to the examiner. Only the culprit can show reaction to the questions, as he alone knows them. The questions are intermixed with irrelevant questions. The reaction is recorded in the Polygram like the other techniques.
Supreme Court on Polygraph
In the year 2010, a bench of the Hon’ble Supreme Court headed by the then Chief Justice of India gave a verdict upon polygraph and other new branches of forensics psychology. The salient features of this judgement, vis-à-vis polygraph interrogation are:
- The polygraph interrogation can be administered only with the consent of the subject before a magistrate.
- Nobody be forced to undergo the test involuntarily.
- The material and other evidence derived from the polygraph interrogation can be used as evidence under the Section 27 of the Indian Evidence Act
- Direct evidence from polygraph interrogation is inadmissible unless supported by other evidence.
- Involuntary interrogation amounts to 3rd degree methods and the court cannot permit such tests.
- The compelling public interest cannot justify ignoring rights against self-incrimination.
- Parliament may consider enacting laws to make the involuntary test admissible in compelling public interest cases.
The National Human Rights Commission, after bestowing its careful consideration on this matter of great importance, has laid down the following guidelines relating to the administration of Lie Detector Tests:
(i) No Lie Detector Tests should be administered except on the basis of consent of the accused. An option should be given to the accused whether he wishes to avail such test.
(ii) If the accused volunteers for a Lie Detector Test, he should be given access to a lawyer and the physical, emotional and legal implication of such a test should be explained to him by the police and his lawyer.
(iii) The consent should be recorded before a Judicial Magistrate.
(iv) During the hearing before the Magistrate, the person alleged to have agreed should be duly represented by a lawyer.
(v) At the hearing, the person in question should also be told in clear terms that the statement that is made shall not be a ‘confessional’ statement to the Magistrate but will have the status of a statement made to the police.
(vi) The Magistrate shall consider all factors relating to the detention including the length of detention and the nature of the interrogation.
(vii) The actual recording of the Lie Detector Test shall be done in an independent agency (such as a hospital) and conducted in the presence of a lawyer.
(viii) A full medical and factual narration of manner of the information received must be taken on record.
 Pg 78, Medical Jurisprudence, Toxicology & Forensic Science for Class Room, Investigation & Court Room With Case Laws by Prof (Dr.) A.S. Deoskar
 Pg 1580-1582, Forensic Science in Criminal Investigation and Trials by B.R. Sharma
 AIR 2010 SC 1974; (2010) 7 SCC 263