Narcolepsy is a chronic hypersomnia characterized by excessive daytime sleepiness (EDS) and manifestations of disrupted rapid eye movement sleep stage (cataplexy, sleep paralysis, and hypnagogic/hypnopompic hallucinations). Mechanisms underlying narcolepsy are not fully understood. Experimental data indicate that the disease is caused by a loss of hypocretin neurons in the hypothalamus, likely due to an autoimmune process triggered by environmental factors in susceptible individuals. Most patients with narcolepsy and cataplexy have very low hypocretin-1 levels in the cerebrospinal fluid. An appropriate clinical history, polysomnogram, and multiple sleep latency test are necessary for diagnosis of the disease. Additionally, two biological markers, i.e., cerebrospinal fluid hypocretin-1 levels and expression of the DQB1*0602 gene, are used. The treatment of narcolepsy is aimed at the different symptoms that the patient manifests. Excessive daytime sleepiness is treated with psychostimulants (amphetamine-like, modafinil and armodafinil). Cataplexy is treated with sodium oxybate (GHB), tricyclic antidepressants, or selective serotonin and noradrenaline reuptake inhibitors. Sleep paralysis, hallucinations, and fragmented sleep may be treated with sodium oxybate. Patients with narcolepsy should follow proper sleep hygiene and avoid strong emotions.