Event-related potentials (ERPs) are scalp recorded electrophysiological responses that are related to an internal cognitive event. This review summarizes recent findings on the effects of neurologic disease and the origin of ERPs obtained in expectancy, attention, memory, and linguistic tasks. Cognitive ERPs allow the physiologic activity of the brain to be analyzed with exquisite temporal resolution. Our understanding of these potentials is incomplete at present. Advances in cognitive psychology and source localization of these surface potentials, however, may result in an increased understanding of both the organization of cognitive processing in the brain and cognitive deficits that result from neurologic disease.