Narcolepsy is a common but underdiagnosed sleep disorder characterized by excessive daytime sleepiness and abnormal manifestations of REM sleep: cataplexy, sleep paralysis and hypnagogogic hallucinations. Sleep onset REM periods (SOREMPs) are diagnostic. Within the last five years the pathogenesis has become clearer: narcolepsy is associated with almost total absence of the neuropeptides hypocretin-1 and -2 in the hypothalamus and CSF. A low level of CSF hypocretin is now recognized as a new diagnostic tool. This review provides updated knowledge of narcolepsy.