Beyond the cerebral impact of cardiac arrest, recent research indicates a high prevalence of neurological disturbances such as delirium and coma among patients admitted to the intensive care unit (ICU). These disturbances, grouped here under the term "encephalopathy," may be overlooked while attention is devoted to reversing life-threatening imbalances in cardiac and pulmonary function. Nevertheless, there is ample evidence that encephalopathy is an independent predictor of mortality. Factors associated with encephalopathy include primary cerebral disorders such as stroke, trauma, and meningitis, or systemic derangements including sepsis, organ failure, and exposure to pharmacological agents and toxins. Although encephalopathy may resolve with treatment of the underlying disorder, there is mounting evidence that cerebral dysfunction persists beyond the acute phase of critical illness. ICU survivors often suffer chronic impairments in cognitive ability, suggesting occult brain injury. The pathogenesis and natural history of encephalopathy, still poorly understood, need further clarification to spur the development of effective preventive and therapeutic interventions.