The face of psychology has undergone a series of changes during a century of growth as a science. Initial concerns with sensory processes, (as, for instance, in the hands of Helmholtz and Mach) and thought (as studies by Külpe, Brentano and James) gave way to investigations of feelings (e.g., Wundt) and motivations (e.g., Freud). The introspectionism of Titchener was succeeded by the factors of Spearman, Thurstone, and Cattell and by the behaviorism of Watson; the Gestalts of Koffka, Köhler, Wertheimer and Metzger were pitted against the learning theories of Pavlov, Hilgard, Hull, Spence, Tolman and Skinner. Each of these faces has left a legacy which can be traced through its descendants and the variety of their modifications, techniques and formal statements of what constitutes psychology, and attests to the vigor of this young science.