Interpersonal relatedness (attachment) and self-definition (separation) are fundamental psychological dimensions that are central to psychoanalytic thought, beginning with Freud, as well as in a wide range of nonpsychoanalytic formulations. These constructs provide a theoretical matrix that facilitates identifying continuities among personality development, variations in normal personality or character formation, concepts of psychopathology, and mechanisms of therapeutic action. The identification of these continuities enables us to consider many forms of psychopathology, not as diseases with assumed but as yet undocumented biological origins, but as distortions that derive from variations and disruptions of normal psychological development. Likewise, the identification of these continuities enables us to consider the relationship of mechanisms of therapeutic action to psychological development more generally. Validation of aspects of these formulations have been found in studies of depression and personality disorders as well as in systematic investigations of therapeutic change.