Psychoanalysis classically viewed dissociation as a defense against oedipal conflict. In recent years attention has shifted from this drive theory model to the causal role of pregenital trauma in multiple personality disorder. Trauma that predates the emergence of symbolizing ego functions appears to predispose to this disorder. The presymbolic child depends upon illusion, a transitional state between reality and fantasy, to master separation. Trauma during this phase replaces illusion with an abiding experience of “primitive agony.” The unavailability of illusion impedes the consolidation of a coherent sense of self; and it fosters the formation of delusions in the effort to preserve a disintegrating reality. Reality sense is impaired. Individuals with multiple personality disorder may be narcissistically invested in their diagnosis, which gives narrative structure to their chaotic unsymbolized traumatic experience. The author recommends postponing reality testing and fostering expression of the unsymbolized traumatic experience in the context of a secure therapeutic relationship.