Delirium has been recognized for the last 3 millennia and is the most common complication found in hospitalized patients aged 65 and older in the United States. However, critical basic science and clinical research did not progress until the DSM III criteria clearly defined delirium 20 years ago. The term delirium then replaced many nonspecific entities, such as acute confusion state, acute brain syndrome, metabolic encephalopathy, and toxic psychosis. This review discusses the epidemiology, risk factors, interventions, causes, management, and outcomes of delirium. The pathophysiology of delirium has the potential to radically alter our management of delirium and is a controversial area of research.