Capgras' syndrome is the delusional belief in the existence of identical "doubles" of significant people in a patient's life, or of the patient himself or herself, or of both. It occurs in both men and women and in a variety of psychotic states. The author presents a review of 133 cases of Capgras' syndrome in the English-language literature, defining the syndrome in the light of Capgras' original reports as well as later descriptions, reviewing the discussions of both organic and functional causes, and suggesting in the context of a new case report a psychodynamic hypothesis that may provide a more adequate understanding and more effective treatment of the syndrome.