Presence of encephalopathy or focal dysfunction is demonstrated by electroencephalograms (EEGs). The reasons for such abnormalities may be broad; however, the EEG can help identify the degree of dysfunction and the regions involved. At the bedside, these electrographic findings can be especially helpful as radiographs do not always show abnormalities in a patient who is clearly altered. The EEG is a very sensitive test to evaluate actual neuronal functioning. This chapter will discuss different patterns identified in abnormal cortical functioning and their clinical implications.