Narcolepsy with cataplexy is a sleep disorder characterized by excessive day-time sleepiness associated with uncontrollable sleep urges and cataplexy (loss of muscle tone often triggered by pleasant emotions). Prevalence is estimated between 1/3,300 and 1/5,000. The age of onset varies between 10 and 30 years old. Narcolepsy is a lifelong disease. The average time between the age of appearance of the symptoms and the diagnosis is 10 years. Other, non specific, clinical signs include hypnagogic hallucinations, sleep paralysis, insomnia, hypnopompic hallucinations and weight gain. The disease is due to loss or impairment of the orexin/hypocretin neurons of the lateral hypothalamus that results in decreased hypocretin-1 levels in the cerebrospinal fluid. An autoimmune origin for the disease is possible as 92% of the patients carry the HLA-DQB1*0602 allele. Diagnosis is based on the typical clinical symptoms. However, a polysomnography, completed with five multiple sleep latency tests (MSLT), is often necessary. The MSLT reveal a mean sleep latency of less than 8 min with at least two episodes of paradoxal sleep. The presence of the HLA-DQB1*0602 marker is a sensitive but not very specific diagnostic criterion. Measurement of hypocretin-1 levels in the cerebrospinal fluid can confirm the diagnosis. Daytime sleepiness associated with attacks of cataplexy allows diagnosis of the disease. In case of atypical or incomplete cataplexy, other causes of sleepiness must be taken into account, such as chronic insufficient sleep, idiopathic hypersomnia (see this term) or narcolepsy without cataplexy (see this term). Treatment comprises stimulant (modafinil, methylphenidate or amphetamine) and anticataplectic drugs (antidepressants or sodium oxybate). First-line treatment of diurnal somnolence is modafinil. Sodium oxybate is efficient for sleepiness, cataplexy and bad quality of night sleep. Narcolepsy can severely disable scholarly and professional performances of the patients. The evolution of the disease is often stable with a frequent improvement of sleepiness and cataplexy, but an aggravation of the poor quality of night sleep, with age.