The EEG is a useful and, at times, an essential test in the evaluation of delirium. In most patients with delirium, the EEG will show diffuse slowing and thus is helpful in differentiating organic etiologies from functional, psychiatric disorders. The degree of the EEG changes correlates with the severity of the encephalopathy so that the EEG may be used to help monitor therapy. In some delirious patients, the EEG may indicate whether the patient is suffering from focal, rather than global, impairment. Furthermore, the EEG is the only test that can identify an ongoing epileptic state (e.g., nonconvulsive status epilepticus) as being responsible for the clinical picture of confusion. Other electrophysiological tests that may prove helpful in the evaluation of delirium, such as computerized EEG spectral analysis, topographic brain mapping, and sleep studies, are briefly reviewed.