This review evaluates attempts to classify and account for suggestibility observed in normal Ss under waking (nonhypnotic) conditions. Factor analytic studies do not confirm the traditional classification of "primary" and "secondary" suggestibility presented by Eysenck and Furneaux. These studies identify at least 3 types of suggestibility: primary (motor), challenge, and imagery (sensory) suggestibility. Intrinsic, stimulus, and response characteristics of suggestibility tests are examined. Suggestibility does not seem to depend on set, expectancy, or situational variables; is not adequately explained by ideomotor action; and is unrelated to the placebo response. State-specific response to suggestion occurs during sleep. Suggestibility may be related to aspects of classical, verbal, and sensory conditioning.Suggestibility has been an important concept in the history of psychology and psychiatry. Apart from being equated with gullibility and persuasibility (Abraham, 1962), the concept of suggestibility has been central to the historical development of hypnosis (Weitzenhoffer, 1953); has been used to explain the placebo response in psychopharmacology (Trouton, 1957); and has been employed as a measure of personality characteristics, particularly neuroticism (Cattell, 1957;Eysenck, 1947). This review evaluates contemporary attempts to classify different types of suggestibility, and examines some frequently postulated mechanisms that appear relevant to current research. A comprehensive review of the suggestibility literature was published by Stukat (1958), and studies prior to the mid-1950s are not reviewed unless considered relevant to the issues raised.This review is concerned with waking suggestion administered to normal populations of subjects. The term "waking" is used in contrast to "hypnotic." Waking and hypnotic suggestibility are conceptually, and quite probably empirically, distinct (Evans, 1966;Hammer, Evans, & Bartlett, 1963). Except