This article examines the validity of polygraphy from the perspective of the science of psychophysiology, which employs physiological measures to study and differentiate psychological processes. The focus is on the version practiced currently by most members of the American Polygraphy Association, the 'control question technique' (CQT). A brief consideration of some critical terms is followed by a description of CQT polygraphy, and then by a review of the literature. We conclude that as a scientific tool, CQT polygraphy is of questionable validity, although it is probably a better-than-chance detector of guilt. Tabular data and 44 references. (Publisher abstract)