Delusional disorder, under its former soubriquet of paranoia, is a venerable diagnosis. Unfortunately both the concept and the diagnosis fell into abeyance in the early part of the twentieth century and have only come back into prominence since 1987, when paranoia – renamed delusional disorder – was revived in DSMIIIR (the revised third edition of the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders). It has subsequently been confirmed in the tenth revision of the World Health Association's International Statistical Classification of Diseases (ICD10, 1992–93) and in DSMIV (1994) and, as will unfold in the course of this book, a considerable world-wide and cross-disciplinary literature on the subject has grown up in recent years.