The multiple sleep latency test (MSLT) is used in the assessment and diagnosis of disorders of excessive somnolence and to evaluate daytime sleepiness in relation to various therapeutic or experimental manipulations, such as administering drugs and altering the length of timing of nocturnal sleep. The repeated measurement of sleep latency across a day provides direct access to the diurnal fraction of the sleep/wake interaction, which is of fundamental concern to the sleep specialist. Objective laboratory documentation of the clinical symptoms of slepiness well as abnormal sleep structure has greatly facilitated the diagnosis of narcolepsy, in particular, and has also been useful to determine the severity of somnolence and therapeutic response in other disorders. At the current level of clinical experience, a diagnosis of narcolepsy or other disorders of excessive somnolence usually has lifelong consequences for the patients, for example, chronic chemotherapy with psychoactive compounds, legal proscription from driving, or surgery. It therefore is incumbent upon the clinical sleep specialist to achieve as much diagnostic precision as possible. The MSLT greatly enhances the accurate diagnosis of disorders of excessive somnolence.