The electrographic features of sleep have been studied intensively using both routine electroencephalography and polysomnography. The two sections of this paper describe and illustrate the major characteristics of sleep physiology and associated clinical electroencephalography. In the first section, criteria for definition of sleep stages are presented and correlated with the phasic and tonic physiological events noted to occur as a function of sleep stage. The phylogeny and ontogeny of sleep are discussed as well as the neurophysiological mechanisms that may underlie sleep control. The second section presents a clinically oriented description of the patterns and events of the sleep electroencephalogram as seen in a normal adult and pediatric population.