The International Classification of Sleep Disorders is the product of 5 years' concerted effort on the part of the Diagnostic Classification Steering Committee of the American Sleep Disorders Association. This committee, chaired by Dr Michael Thorpy, worked in collaboration with sleep researchers, clinicians, and associations from all over the world to produce a definitive classification system of sleep disorders. The broad scope of this multispecialty and multinational project is readily apparent. The revision of the original Diagnostic Classification of Sleep and Arousal Disorders, published in 1979, involved surveying members of US and international professional sleep societies, and collating, evaluating, and responding to identified concerns. The result is a comprehensive and usable classification system. In contrast with the four clusters of disorders of the original nosology, primary sleep disorders are now organized into two major groups: dyssomnias and parasomnias. The dyssomnias include those disorders producing a complaint of insomnia or daytime