Paranoia and its related disorders were regarded as an important group of psychiatric illnesses until the early part of the twentieth century. Then, because of prevalent classification practices – particularly the tendency to overdiagnose schizophrenia – the diagnoses of paranoia and paraphrenia virtually died out. In 1987, paranoia was revived by DSMIIIR and was renamed ‘delusional disorder’: as such, it currently is the only officially recognized member of the old paranoid disorder clustering.

Although the diagnosis disappeared, the illness and its sufferers did not. The result was both an inappropriate ‘lumping’ of cases of delusional disorder into other categories, most usually schizophrenia, and an extraordinary ‘splitting’, in which cases of paranoia/delusional disorders were recognized for some secondary feature, but their true diagnosis was ignored. The latter especially has meant a profoundly scattered literature and a great deal of confusion as to what is delusional illness and what is not.

This book is an attempt to define more clearly the concept of paranoia/delusional disorder and to gather the shards of the current body of knowledge into a more coherent whole. It also tries to define the limits of delusional disorders and to dispel some of the confusion which still exists when trying to exclude vaguely similar illnesses. At the same time, a strong effort is made to point out that paranoia/delusional disorder is not the only ‘delusional disorder’: for example, paraphrenia and delusional misidentification syndromes (DMS) are strong candidates for inclusion in an expanded category.