The diagnosis of dissociative identity disorder, formerly known as multiple personality disorder, remains controversial, despite its inclusion as an established diagnosis in psychiatry’s Diagnostic and Statistical Manual (DSM-IV). This book consists, first, of the detailed description of the treatment of a patient whose syndrome of DID emerged in the course of therapy, by a psychoanalyst who entered into the therapy as a complete skeptic. The therapist’s willingness to explore the meaning and reality of the dramatic and at times bizarre presentation of this disorder in a prolonged and difficult treatment was accompanied by an eventual successful outcome. Using this case as a reference point, the author then provides a contemporary exploration of the literature about the authenticity of this diagnosis and approaches to its treatment.