Excessive Sleepiness, or hypersomnia, is one of the most common sleep complaints. In this issue, Dr. Alon Avidan of UCLA brings together a set of articles that offer a completely updated overview of hypersomnia, from neurophysiology of sleepiness and wakefulness to quality of life issues and public health. The main focus of the issue is the diagnosis and treatment of hypersomnia, including objective and subjective measurement of sleepness, biomarkers of sleepiness, narcolepsy, and hypersomnia in medical, neurological and psyhchiatric comorbidities. Excessive sleepiness among specific patient populations (children, elderly) and periodic hypersomnia are discussed. Pharmacotherapy of hypersomnia is given special attention, as are behavioural treatments.