The classification of diseases, or the branch of medicine that deals with naming and classifying diseases.Nosology has an extremely important place in psychiatry, as it does in general medicine. It is not merely a matter of labeling or pigeonholing disorders, but of discovering their most indicative symptoms and the grouping of symptoms into syndromes. Any psychiatric condition must be recognized and correctly diagnosed before appropriate therapy can be applied and a secure prognosis made. It is not always easy, however, to make a diagnosis in psychiatry since the same symptoqps are frequently found in different disorders. Moreover, many disorders cannot be confined to one or another category, and a “mixed” diagnosis must often be made.In addition to its important role in diagnosis and treatment, classification plays a major role in developing prevention programs and research projects.It must be recognized, however, that a nosological approach can be carried to the extreme, as it was during the “descriptive era” in the nineteenth century, when the various disorders were considered separate and distinct. At that time greater emphasis was placed on naming diseases and describing symptoms than on searching for all possible causal factors and attempting to interpret the inner meaning of symptoms for the particular patient. In the dynamic psychiatry of today these aspects are fully recognized.Emil Kraepelin (1856-1926) is considered the greatest nosologist in the history of psychiatry. He accumulated a vast mass of clinical observations, and defined and classified practically all the severe disorders now known to exist. The study of psychiatric nosology, however, is a continuing process and is responsible for the nomenclature and classification published by the American Psychiatric Association in 1952 and used in this encyclopedia. Among the newer categories are the various personality pattern and trait disturbances, and the transient situational personality disorders that occur in both childhood and adulthood. See KRAEPELIN, DESCRIPTIVE PSYCHIATRY, DIFFERENTIAL DIAGNOSIS.