Altered mental status is a common pathological entity in critically ill patients and particularly in those with preexisting cerebral injury. In the neurological critical care unit, the prevalence of altered mental status is especially high because of the inherent nervous system disease of these patients. Altered mental status can be crudely divided into encephalopathy and delirium. Although often used interchangeably, the 2 pathological entities have subtle differences in etiology and presentation. This is a review of delirium and encephalopathy in the neurological critical care unit.