The electroencephalogram (EEG) is an important diagnostic tool in coma. Although comatose patients may be similar on neurological examination, the EEG reveals a wide range of abnormalities. For metabolic and septic encephalopathies, the "anesthesia model" is a useful analogy. The EEG is very sensitive to the depth or severity of brain dysfunction in coma as well as the direction of the process if serial tracings or continuous recordings are used. While the EEG is rarely specific for the etiology of coma, it may help determine the class or general category of disease process. In conditions capable of causing neuronal death, e.g., anoxia-ischemia, the EEG can be of prognostic value.