Today excessive daytime sleepiness is recognized as a pathology of its own. It is both frequent and at the root of numerous complications for the individual and the society. It is usually identified with much delay, due to its progressive development and the fact that the patient gets used to it and does not foresee its deleterious consequences. The positive diagnosis relies on the symptoms reported by the patient and his relatives, and the use of introspective and behavioural subjective scales and objective tests. The aetiologic diagnosis is founded on a thorough clinical interview and a physical and psychological examination, followed, if necessary, by laboratory tests conducted in special units referred to as sleep disorders centers. The aetiologies include behaviourally induced excessive daytime sleepiness, proper sleep disorders, psychiatric or medical disorders, and intake of medications or substances. New treatments are currently available or in preparation in the case of narcolepsy.