Delusion has always been a central topic for psychiatric research with regard to etiology and pathogenesis and to diagnosis, treatment, and forensic relevance. Throughout the history of psychiatry as a scientific discipline, there has been dissent on the issue of whether chronic delusion is a nosological entity of its own or just a specific type of another mental disorder, e.g. schizophrenia, mania, or personality disorder, and there already is a considerable literature on this. This article seeks to elucidate the central lines of thought that have governed the scientific debate on delusions and delusion-associated phenomena since the early 19th century. Special attention is given to the practical relevance of these theoretical considerations for forensic questions and psychiatric research. Due to the complex features of delusions, research in this area may well become paradigmatic for many other complicated psycho(patho)logical phenomena, e.g. consciousness, hallucinations and psychotic depression.