The development of organized sleep/wake states is a major feature of the neonatal period and developing infant. Although subsequent changes in the sleep/wake cycle with age are less profound, they represent predictable physiologic changes occurring as a function of age. Similarly, there are characteristic electroencephalography (EEG) patterns seen as a function of maturity in the neonate and developing child. These EEG changes continue to evolve into adulthood, offering an electrophysiologic marker of brain development.