Of all the techniques attempted to determine truth of deception, only polygraphy has stood the test of time. Polygraphy use is increasing in courts, and in many jurisdictions, the testimony of polygraphists is routinely admitted on a stipulated basis. Attorneys are also relying more heavily upon polygraphy in both criminal and civil cases. Since no one of the theories advanced to explain the effectiveness of the polygraph technique is adequate in itself to explain lie detection in the varying situations, it must be assumed that there is no one explanation. For laboratory research, generalized excitement may be the main factor, while in field investigations, it may be fear of the consequences. The polygraph mechanism measures physiologic changes in the subject during the testing but does not deduce deception. The polygraphist infers deception from the measurements of the polygraph under the assumption that the physiological responses result from fear reactions associated with deception. The effectiveness of the polygraph depends on the physical, mental, and emotional state of the subject; the instrument being used; the testing environment; and the expertise of the polygraphist. The polygraph procedure consists of the distinct phases of data gathering, pretest interview, testing, and the posttest interrogation. Of all the field validity studies reported, only one has demonstrated a low level of accuracy for the polygraph. Reliability studies have been conducted much less frequently than validity studies, but the findings tend to be consistently high. Laboratory settings have demonstrated that polygraph accuracy can be greatly reduced when the subject presses his/her toes against the floor, engages in exciting fantasies, or has been subjected to hypnotic suggestions. It can be expected that more courts will admit polygraph testimony over objection and that there will be a continuing trend to make use of this procedure in a wider variety of cases. A total of 100 footnotes are provided.