Every serious school of psychotherapy has its own theory—often only vaguely formulated—concerning the active ingredients in psychotherapy. Many of these theories are presented as mutually exclusive. The author presents an overview of some of the important, primarily psychoanalytically founded, theories of the factors in individual psychotherapy that are responsible for inducing change. It is impossible to pinpoint any single factor that is crucial in every therapy. What is needed is a nondogmatic, multiple-factor model that successfully incorporates the knowledge obtained from the many existing theories of psychotherapy-induced change. In practice, it is often difficult to maintain the traditional distinction between specific and nonspecific factors, just as it is difficult to distinguish the roles played by purely therapeutic factors—relating to the technique of the therapist—and by extra-therapeutic factors. The author also addresses the epistemological status of the various claims put forward, by the many different theories of psychotherapy, concerning the active ingredients in psychotherapy.