The intensivist should think of delirium, or acute central nervous system dysfunction, as the brain's form of "organ dysfunction.'' Delirium is extremely common in intensive care unit (ICU) patients due to factors such as comorbidity, critical illness, and iatrogenesis. This complication of hospital stay is extremely hazardous in older persons and is associated with prolonged hospital stays, institutionalization, and death. Neurologic dysfunction compromises patients' ability to be removed from mechanical ventilation or achieve full recovery and independence. Yet ICU nurses and physicians are usually unaware of the presence of hypoactive delirium and only recognize this disturbance in agitated patients (hyperactive delirium). More importantly, there are few studies that have included ICU patients in the assessment or prevention of delirium. This article reviews the definition and salient features of delirium, its primary risk factors, a newly validated instrument for delirium assessment that is being developed for ICU nurses and physicians, and pharmacological agents associated with the development of delirium and used in its management.