Interictal electroencephalogram (EEG) provides useful information about the presence of nonepileptiform neurophysiological dysfunction. When abnormalities are encountered, they are not specific for an underlying etiology, and as such represent abnormalities without further differentiation of the pathological process. This chapter discusses and focuses on generalized and focal nonepileptiform abnormalities. Diffuse slowing on the EEG may have various morphologies, and occur intermittently or continuously, to reflect abnormal cerebral function. Low-voltage EEG is typically associated with diffuse slowing of the background rhythm. Focal abnormalities on the EEG provide electrographic evidence of a localized abnormal cerebral function. Intermittent irregular slowing has a low correlation with an underlying lesion compared to focal slowing that is continuous. EEG can reveal many types of nonepileptiform abnormalities corresponding to varied cerebral dysfunctions occurring at any age. Reactivity of nonepileptiform abnormalities is an important feature that helps assess the cause and prognosis of the clinical condition.