Dissociative identity disorder (DID) used to be called multiple personality disorder. Someone diagnosed with DID may feel uncertain about their identity and who they are. They may feel the presence of other identities, each with their own names, voices, personal histories and mannerisms. The main symptoms of DID are:

Dissociative identity disorder was formerly called multiple personality disorder. The symptoms reflect the breakup of a single self rather than a multiplication of selves. The fragmentary selves seem to have strikingly different personal histories, behavior patterns, friends and companions, beliefs, habits, values, even voices and facial expressions. They alternate in assuming complete or partial control of awareness, attention, and action, with transitions occurring mainly under stress. People with dissociative identity disorder often have a great variety of psychiatric symptoms. The complicated symptoms of dissociative identity disorder are often treated with long-term individual therapy. Focusing on safety first, therapists may begin by helping a suspicious or demoralized patient resolve a state of crisis, discussing practical problems and providing support.

Dissociative Identity Disorder is sometimes incorrectly believed to be a "new" diagnosis, but it has a long history of recognition, and has been part of the Diagnostic and Statistical Manual for Mental Disorders since its first edition was published in 1952.

Impairment in dissociative identity disorder varies widely. It may be minimal in highly functioning patients; in these patients, relationships (eg, with their children, spouse, or friends) may be impaired more than occupational functioning.