Symbiosis is a central theoretical construct in a number of psychoanalytic formulations of personality development, yet its validity has recently been challenged. On the basis of a model of development that views self identity as emerging through a dialectical interaction of two primary developmental lines—attachment and separateness—we suggest that the intrapsychic state of undifferentiation denoted by the term symbiosis refers to two distinct kinds of experience. In terms of attachment it refers to an experience of merger; in terms of separateness, to fusion. An examination of these two dimensions clarifies some of the contemporary challenges to the concept of symbiosis and furthers the understanding of the role of the varied experiences of symbiotic unity in development as well as in psychopathology.