The sudden surge of reports on multiple personality in the 1970s raises serious questions as to whether multiple personality is the rare phenomenon it has been believed to he, or whether it is a relatively common condition which is misdiagnosed with a high order of frequency. Recent research in ego psychology, particularly into the etiology of the borderline disorders, sheds considerable light on the origins of multiple personality which has traditionally, and probably mistakenly, been considered a wholly hysterical disorder. The author proposes a detailed theory of the genesis of multiple personality, based on the existing ease literature and on contemporary theories of ego development. The issues of authenticity, nosology, diagnosis, personality structure, neuropathy, etiology, and treatment are considered at length.