Delirium is a common syndrome in hospitalized medical and surgical patients and in those seen in emergency departments. It is particularly common in the demented elderly patients. To prevent it, for example by avoiding polypharmacy in the elderly, is important. It is most important to diagnose delirium early and to search for its cause without delay as this may be life-saving in some patients and may help avoid injury in others. This syndrome, long neglected by investigators, calls for vigorous research to establish its epidemiology, pathogenesis, and long-term outcome.