Narcolepsy is a chronic neurological sleep disorder caused by hypocretin neuron loss, resulting in excessive daytime sleepiness, disturbed nocturnal sleep, and intrusions of aspects of rapid eye movement sleep in wakefulness, such as cataplexy, sleep paralysis, and hypnopompic/hypnagogic hallucinations. Narcolepsy disrupts the maintenance and orderly occurrence of the wake and sleep stages. Cataplexy is a highly specific symptom of narcolepsy, but many other symptoms can be observed in a variety of sleep disorders. The diagnosis of narcolepsy type 1 requires a history of excessive daytime sleepiness and one of the following : 1) a low cerebrospinal fluid hypocretin-1 level or 2) cataplexy and a positive multiple sleep latency test result. The diagnosis of narcolepsy type 2 requires a history of excessive daytime sleepiness and a positive mean sleep-latency test result. The mean sleep-latency test must be preceded by nighttime polysomnography to exclude other sleep disorders and to document adequate sleep. The mean sleep-latency test result can be falsely positive in other sleep disorders, such as shift work, sleep apnea, or sleep deprivation, and it is influenced by age, sex, and puberty. Modafinil and armodafinil can reduce the excessive daytime sleepiness without many of the side effects associated with older stimulants. Although there is no cure for narcolepsy, the treatments are often effective and include both behavioral and pharmacologic approaches.