Within the last decade, multiple personality disorder has been diagnosed, treated, and studied with increasing frequency. A growing body of literature suggests that this condition is not a rarity but that its manifestations, often coexisting with or obscured by other phenomena, frequently go unrecognized for years within the mental health care delivery system. The author reviews recent advances in the understanding of the etiology, diagnosis, and treatment of multiple personality disorder. The thrust of the contemporary literature is that it is a difficult-to-diagnose condition that usually follows severe child abuse, that it is most parsimoniously understood as a chronic dissociative posttraumatic stress disorder, and that it has an excellent prognosis when intensive and prolonged psychotherapy with an experienced clinician is available.