A brief overview is given of the history of psychiatric nosology during the last 150 years. The varying role of genetics for theories of nosology is described. Classifications of mental disorders, more or less based on nosological concepts, are mentioned, as well as the recent emphasis on operational tools for classification. Conflicts between nosology and modern classifications are mentioned. The place of the schizophrenic syndrome in relation to nosological theories is described, including the questions of schizophrenia being a disease entity, or several entities, or even a part of a continuum, or just a symptomatically characterized reaction form.