Psychoanalysis began with Freud's investigation of hysteria, first at Charcot's clinic in Paris, then in Vienna in collaboration with Breuer, and finally alone. Among the momentous discoveries resulting from these early studies was the fact that certain discrete childhood experiences crucially influenced the subsequent lives of his patients. These experiences, to which he gave the name “psychical traumas,” first impressed him because of the central and specific role they played in the causation of illness and the formation of symptoms. As a clinician, Freud was concerned with trauma because of its pathogenicity. As a theoretician, he was concerned with the intrapsychic events and mechanisms operative in trauma at the time of its occurrence. Here his formulations centered on the concept of the stimulus barrier.