Various authors and researchers have proposed that polygraph countermeasures are "methods a guilty test subject uses to manipulate the test recordings to render a truthful outcome." The taxonomy presented in this monograph is based on two criteria. The first and overarching principle is that countermeasures that use mechanisms in common are grouped together. The second criterion is that classification be based solely on the primary characteristics of the countermeasure. Based on these two criteria, the literature suggests that all polygraph countermeasures can be grouped into four categories: physical countermeasures, mental countermeasures, pharmacological/chemical countermeasures, and behavioral countermeasures. As a rule, any method that involves muscular movement as its central feature can be considered a physical countermeasure. Some countermeasure movements could include the increasing or releasing of muscle tension, actions that induce pain, muscular activities that deplete the body's energy resources, and alterations of the normal respiratory cycle. Mental countermeasures are those that draw upon psychological manipulations exclusively in order to alter or mediate the physiological responses concomitant to deception. Mental countermeasures can be further subdivided into the following techniques: hypnosis, biofeedback, placebos, desensitization, mental dispositions, rationalization, dissociation, and cognitive overloading. Pharmacological/chemical countermeasures rely on drugs to heighten or dampen physiological arousal during the polygraph examination. The principal function of behavioral countermeasures is to convince the examiner that the subject is not deceptive, irrespective of reactions shown on the strip charts. Another function is to affect the conduct of the examination so that the data will be inadequate to render a decision. The main defenses against the various categories of countermeasures are described. 62 references