Excessive daytime sleepiness (EDS), the primary complaint of patients seen in sleep clinics, affects up to 12% of the general population. The effects of EDS can be debilitating and even life threatening. Patients with EDS may exhibit psychosocial distress, decreased work or school performance, and increased risk for accidents. The differential diagnosis of EDS requires objective assessments, such as polysomnography and the Multiple Sleep Latency Test. There are four major causes of EDS: (1) central nervous system (CNS) pathologic abnormalities, such as narcolepsy and idiopathic CNS hypersomnia; (2) qualitative or quantitative sleep deficiencies, such as sleep apnea and insufficient nocturnal sleep; (3) misalignments of the body's circadian pacemaker with the environment (eg. jet lag or shift work); and (4) drugs, which can increase sleepiness either therapeutically or as a side effect. Depending on etiology, management strategies for EDS include extension of time in bed, naps, surgery, various medical devices (eg, oral appliances, continuous positive airway pressure), and pharmacotherapy. Pharmacotherapy is generally achieved with stimulants, such as amphetamine sulfate, methylphenidate, and pemoline or newer, safer compounds like modafinil.