Dissociative identity disorder (DID) is a complex, posttraumatic, developmental disorder for which we now, after four decades of research, have an authoritative research base, but a number of misconceptualizations and myths about the disorder remain, compromising both patient care and research. This article examines the empirical literature pertaining to recurrently expressed beliefs regarding DID: (1) belief that DID is a fad, (2) belief that DID is primarily diagnosed in North America by DID experts who overdiagnose the disorder, (3) belief that DID is rare, (4) belief that DID is an iatrogenic, rather than trauma-based, disorder, (5) belief that DID is the same entity as borderline personality disorder, and (6) belief that DID treatment is harmful to patients. The absence of research to substantiate these beliefs, as well as the existence of a body of research that refutes them, confirms their mythical status. Clinicians who accept these myths as facts are unlikely to carefully assess for dissociation. Accurate diagnoses are critical for appropriate treatment planning. If DID is not targeted in treatment, it does not appear to resolve. The myths we have highlighted may also impede research about DID. The cost of ignorance about DID is high not only for individual patients but for the whole support system in which they reside. Empirically derived knowledge about DID has replaced outdated myths. Vigorous dissemination of the knowledge base about this complex disorder is warranted.