The editors first describe the principal symptoms of the Capgras delusion, the Frégoli delusion, the delusion of intermeta-morphosis and the delusion of subjective doubles before developing the argument that it would be appropriate for international psychiatric diagnostic systems to include these disorders. Furthermore the similarity between them, the reduplicative paramnesias and déjà and jamais vu are pointed out. By stressing a symptom-based approach it is possible to examine psychiatric, neurological and medical cases as arising from the same underlying disturbance in cognition function. The relationship between delusions of misidentification and other delusions is also touched upon.