This chapter presents a phenomenological method for making measurements of the extent of development of variables important to an understanding of hypnosis. It provides a survey and comparison of the several methods designed to measure hypnotic depth, including the phenomenological. The chapter describes the conditions under which retrospective phenomenological descriptions may be presumed to be accurate. The traditional and theoretical approaches yield differing operational definitions of hypnotic depth. Three general methods have been developed in psychology for the measurement of hypnotic depth. These three may be briefly labled the behavioral, clinical, and subjective methods. The theoretical approach provides a different kind of operational definition and consequently a different conception of the meaning of hypnotic depth than is provided by the traditional approach. The three dimensions of hypnotic depth are: trance, nonconscious involvement, and archaic involvement.