Although epilepsy is defined by the occurrence of spontaneous epileptic seizures, a large body of evidence indicates that epilepsy is linked to a spectrum of behavioral, psychiatric, and cognitive disorders as well as to sudden death. Explanations for these associations include the following: (1) The effects of structural lesions that may impair the functions subserved by the regions of the brain involved in the lesion. (2) The effects of seizure activity that may begin well before a clinical seizure occurs and may persist long after it is over, raising questions about what truly constitutes "interictal." In addition, encephalopathic effects of epilepsy in infancy during critical periods in development may be particularly severe and potentially irreversible. (3) Shared mechanisms underlying seizures as well as these other disorders in the absence of structural lesions or separate diseases of the central nervous system (CNS). Epidemiologic and clinical studies demonstrate the elevated risk of cognitive, psychiatric, and behavioral disorders not just during but also prior to the onset of epilepsy (seizures) itself. These may outlast the active phase of epilepsy as well. The mounting evidence argues strongly for the recognition of epilepsy as part of a spectrum of disorders and against the notion that even uncomplicated epilepsy can a priori be considered benign.