Narcolepsy is a chronic neurologic disorder characterized by excessive daytime sleepiness, cataplexy, and premature onset of rapid eye movement sleep. It can be differentiated from other disorders causing daytime drowsiness by its clinical symptoms and by sleep laboratory studies. The disorder usually begins in adolescence and remains present throughout life. Genetic susceptibility to narcolepsy is closely associated with specific HLAs that indicate the existence of a gene in the region of the major histocompatibility complex on chromosome 6 that increases susceptibility to narcolepsy. Neurochemical studies of human and canine narcolepsy have demonstrated disturbed monoaminergic and cholinergic function that may account for impaired regulation of rapid eye movement sleep, but the link between these abnormalities and the genetic factors is still unknown. Treatment of sleepiness with stimulants and cataplexy with tricyclic antidepressants leads to substantial improvement but does not fully resolve symptoms in most patients.