The existing knowledge about sleep disorders is systematized in the International Classification of Sleep Disorders by the criterion of pathophysiological mechanisms. The classification is to serve both diagnostic and epidemiological purposes. Four groups of disorders have been distinguished: (1) Dyssomnias, (2) Parasomnias, (3) Sleep Disorders Associated with Psychiatric and Medical Diseases, (4) Proposed Sleep Disorders. Dyssomnias and Parasomnias include major, primary sleep disorders. Particular sleep disorder categories are discussed in the classification in detail and from many viewpoints (symptoms, course and prognosis, underlying pathological mechanisms, complications, sleep EEG and other laboratory tests results, differential diagnosis, references). Objective assessment of typical symptoms is based on polygraphic recordings of sleep and wakefulness.