Delirium is a common condition in hospitalized older patients. Often, a report of a "change in mental status" is the reason geriatric patients are sent to the emergency room for evaluation, although delirium also can develop after admission. Delirium is a marker of underlying medical illness that needs careful workup and treatment. The condition can be iatrogenic, resulting from prescribed medication or a surgical procedure; most often, it is the consequence of multiple factors. Delirium can be expensive, because it increases hospital length of stay and overall costs-particularly if the patient is discharged to a nursing facility, not to home. Patients with delirium are at higher risk of death. Delirium often goes unrecognized by physicians and nursing staff, and is not documented in medical records. Educating the medical staffon the identification and management of delirium is a key role for consulting psychiatrists.