Before diagnosis there is the process of differential diagnosis in which the clinician extracts the probable from the possible and the likely from the unlikely. One cannot make a diagnosis without excluding others – no case is ever that black and white.

Since delusional disorder is still an unfamiliar concept to many mental health practitioners we have to alert them to its existence and teach them how to recognize it. We also have to remind them of those other illnesses which may bear some resemblance to it and underline the significant differences between them and it. In Chapter 2, there is mention of several conditions characterized by delusions and the main features which distinguish them from delusional disorder. Here, we wish to amplify some of these distinctions.

This Part comprises two chapters. In Chapter 11, there is a succinct but relatively detailed consideration of two psychotic disorders which are capable at times of being mistaken for delusional disorder, and which are not particularly well-described in the standard literature.