The most important indication for electroencephalography (EEG) in critically ill patients is to evaluate fluctuating or persistently abnormal mental status (or other focal neurological deficits) that cannot otherwise be explained. Commonly, these symptoms are a manifestation of physiological diffuse cerebral dysfunction (encephalopathy), or they may be due to seizure activity without apparent clinical manifestations. Such “nonconvulsive” seizures (NCS), that may only be detected by EEG, occur in at least 8–10% of critically ill patients. Continuous or frequent NCS is called nonconvulsive status epilepticus (NCSE), and may result in secondary neurological injury, including neuronal death or alteration of neuronal networks. Left untreated, NCSE can become increasingly refractory to treatment. EEGs may be indicated in acute brain injury to detect seizure activity. They are useful in monitoring the depth of anesthesia and in the management of refractory status epilepticus. EEGs may also be used in the intensive care unit to characterize paroxysmal clinical events and in prognostication after cardiac arrest or determining brain death.