Identity is not a Freudian concept. Theoreticians have defined it in very different ways: as a structure that accounts for narcissism and is part of the ego; as the ability to remain the same despite changes; as a feeling of continuity; or as the sum of representations of the self.

The importance of the notion of identity in the United States is related to its use in ego-psychology, which considers the ego as a relatively autonomous and potentially conflict-free structure. Many theories of identity adapt a portion of Freud's view of the ego. Alongside the Freudian ego, which is a structure defined by its functions, another ego—or identity related to identifications—is posited (whether inside or outside ego-psychology) and conceived of as the outcome of a process of individuation.

The first mentions of the importance of the concept of identity for clinical practice and psychopathology date from the nineteen-fifties. When...