American psychiatrists and psychologists have long been close colleagues and fierce rivals. There is no better illustration of this polarized relationship than the chronic tug-of-war over psychotherapy. Both groups laid claim to psychotherapy-whatever it was and however it was practiced. Psychiatrists attempted to monopolize psychotherapy despite its ambiguous status as an essential component of the healing arts. After the war, psychologists pressed for a share on the basis of their qualifications and competence, but struggled to overcome the limitations imposed by medical envy. This story lays bare the crucial function of tools and techniques for defining the identity and the boundaries of a science-based profession.