After a review of foundational contributions to the concept of identity, including Erikson's, the author discusses the research methods and findings of the Personality Disorders Institute of the Joan and Sanford I. Weill Medical College of Cornell University regarding the concepts of normal identity and identity diffusion, toward an elucidation of the psychopathology of personality disorders—their etiology, diagnosis, and treatment. The application of an object relations theory model to analyze the development of identity clarifies the relationship of individual identity with the social and cultural frame that influences identity formation and may amplify the effects of pathological identity development. Detailed excerpts are presented from a diagnostic structural interview at the Personality Disorders Institute.