Psychoanalysis has long lost its historical influence on U.S. academic psychiatry. Psychoanalytic theory, however, provides us with a rich and remarkably comprehensive knowledge of human development, both normative and pathologic. This article describes a psychoanalytic concept that enriches our understanding of the mind and its disorders: Freud's structural hypothesis. This core concept provides a theoretical foundation for understanding the clinical features of both neurotic and personality disorders. It also informs a psychodynamic psychotherapy.