This review discusses recent developments in classifying the sleep disorders. Several classification systems are presented, and the Diagnostic Classification of the Sleep and Arousal Disorders (DCSAD), published by the Association of Sleep Disorders Centers in 1979, is used as the basis for reviewing their differences. The four main sections of the DCSAD are presented: the disorders of initiating and maintaining sleep, the disorders of excessive somnolence, the sleep-wake schedule disorders, and the dysfunctions associated with sleep, sleep stages, or partial arousals. Disorders discovered since the publication of the DCSAD are mentioned along with the new nomenclature of the International Classification of Sleep Disorders, due to be published in 1990. References are given to the original clinical descriptions and nomenclature.