Narcolepsy is best characterized as a disorder of the regulation of sleep and wakefulness, resulting in a variety of symptoms such as excessive daytime sleepiness (EDS), cataplexy, hypnagogic hallucinations (HH), sleep paralysis, and disturbed nocturnal sleep. This chapter focuses on narcolepsy with cataplexy and narcolepsy without cataplexy. Cataplexy is characterized by a sudden bilateral loss of muscle tone, with preserved consciousness, elicited by emotions. Narcolepsy with cataplexy is diagnosed according to the criteria of the International Classification of Sleep Disorders (ICSD-2). The chapter summarizes the differential diagnoses of EDS and cataplexy. Cataplexy and sleep paralysis are both regarded as expressions of the atonia that physiologically occurs during REM sleep, occurring during wakefulness. Two treatment modalities have proven to be effective: behavioral modification and pharmacological therapy. Pharmacological treatment is supplementary to behavioral advice and should be tailored individually.