In 1882 the French psychiatrist Jules Cotard first used the term Nihilistic delusions (ND) to describe the beliefs of patients who denied their existence. Together with other symptoms, these bizarre delusions have since been regarded as a central feature of "Cotard's Syndrome". A century later, the existence, aetiology and relevance of this syndrome to modern psychiatric practice has been questioned. In this thesis the author reviews the literature on ND and sets this against the background of earlier work published on hypochondriacal delusions (HD), and later studies of organic brain syndromes in which the existence of body parts is denied. A study is described in which two groups of patients admitted over one year, to five psychiatric units, who expressed ND (n=15) or HD (n=16), were examined; the admission incidence rates were 3.4 and 4.3, per 1000 admissions, respectively. ND patients were predominantly female and middle aged while the HD group were mostly male and, significantly older. The majority of patients in both groups were clinically depressed, though on PSE testing, a large number of the ND patients had a non-affective psychosis. The pattern of psychiatric illness and response to treatment was similar in the two groups, ECT being an integral part of successful treatment regimes. Data from psychometric testing and CT brain scans failed to support the theory that ND and Cotard's syndrome have an organic aetiology. The preservation of the concept of Cotard's syndrome is argued for, though it is suggested that the illness should more correctly be regarded as schizophrenic, rather than depressive, in type.