Capgras' delusion has captured psychiatrists' imaginations, but the clinical features of the delusion have rarely been studied and presented systematically.The present study systematically reviews all case reports on Capgras' delusion in the English language in order to better understand differences between organic and functional aetiologies.All medical and psychiatric databases were searched, as were the bibliographies of published case reports, narrative reviews, and book chapters.A total of 258 cases were identified from 175 papers. Functional Capgras' delusion was more associated with a wider variety of imposters; multiple imposters; other misidentification syndromes; auditory hallucinations; other delusions; and formal thought disorder. Organic cases were associated with age; inanimate objects; memory and visual-spatial impairments; right hemispheric dysfunction; and visual hallucinations. Executive dysfunction and aggression were associated with both types.Specific features of the -Capgras' delusional content and associated signs point to either organic or functional aetiology. The delusion is more amorphous than many theorists have supposed, which challenges their explanatory models.