The Rorschach is based upon the interaction of three fundamental psychological concepts: set, projection, and perception. Operationally, although definitions vary widely, according to Gibson [10], set may be defined as a state of readiness which causes an organ-•ism to respond to a stimulus in a potentially prescribed manner. Under the heading of sets may be included needs, drives, motives, etc., as determining tendencies. Projection is too wellknown to discuss here. The fact that perception is based upon already existing underlying needs has been amply demonstrated [6,15,16,17], These assumptions are inherent in the rationale of projective devices, especially of the Rorschach, for the fundamental needs, drives, wishes, etc. are contained within the individual as pervasive sets which are projected outwards. Thus, if the Rorschach is to be considered as a valid diagnostic clinical instrument, it is the underlying dynamic tendencies (or sets) which must determine the responses of the subject, not superficial conscious "sets" due to any immediately preceding experience.In reviewing the literature, one is struck by the paucity of material concerning the experimental study of the role of set with the Rorschach. A number of experiments have been reported in which the set of the subject is controlled either by the instructions of the experimenter or by changing the affective tone of the testing situation. Fosberg [9] found that instructions to give best' or worst impressions had no significant influence on the basic Rorschach picture. Levine, Grassi, and Gerson [12,13] found marked changes in the Rorschach of a single subject, corresponding to hypnotically induced moods. Bergman, iThe authors are indebted to Dr. Anne Roe for her valuable suggestions and criticisms.