What should clinicians know about sleep disorders? This article briefly introduces the reader to sleep medicine and succinctly reviews common sleep disorders. First, the authors describe the diagnostic tools unique to sleep medicine: the over-night polysomnogram and the multiple sleep latency test. Next, the authors review essential features of a subset of the sleep, described in the International Classification of Sleep Disorders-Revised, that sleep disorder specialists commonly evaluate, diagnose, and treat. The disorders reviewed include the intrinsic and circadian rhythm subsets of the dyssomnias group and the parasomnia group of sleep disorders. The authors identify the core signs and symptoms, polysomnogram findings, multiple sleep latency test findings, and treatment of these disorders.