Dissociative disorder is both an old and a new mental disorder. Its forerunner, hysteria, was a poorly understood and often grossly misconceived notion that might date back as far as human societies existed, but it has been treated as a new disorder since it emerged in the DSM-III. A century ago, Sigmund Freud and Pierre Janet, two giants in the history of dynamic psychiatry, were deeply influenced by dissociative phenomena, and created their own theory of human mind with very different orientations. The reemergence of dissociative disorders in the current psychiatry has several implications. It helps clinicians understand mental conditions that they did not know how to define and classify based on the traditional psychiatry. It also allows clinicians to identify a population that was misdiagnosed with other diagnostic categories, such as borderline personality disorder and schizophrenia.