ABSTRACT.In neurosurgery today, the EEG is still an important and useful diagnostic tool, particularly in the assessment of the cerebral state and its dynamic changes in the comatose patient. The totality of generalized EEG changes, like amplitude, frequency, or the development of specific EEG patterns, already permits a good estimation of the depth of coma, whereas focal changes are hardly expressive. Particular attention should be paid to alpha activities in the coma, which need to be well differentiated, since they may constitute completely different phenomena of various causes and prognoses. The spontaneous EEG frequently provides prognostic indications, but long-term monitoring with frequency analysis should also be performed to diagnose the existence of autorhythm.The changes of the EEG in comatose patients in response to stimulation by exogenous acoustic, nociceptive, and visual stimuli, and also by medication, may yield additional information. The administration of centrally active drugs such as barbiturates or benzodiazepines is able to reveal the capacity of the brain for fundamental physiological EEG reaction, thus providing a test for the integrity of cerebral structures despite loss of consciousness. This type of functional EEG therefore seems to deliver quite exact information on the extent of irreversible damage and on the prognosis.