Delirium is a mental disorder characterized by disturbances in consciousness, orientation, memory, thought, perception, and behavior, of acute onset and fluctuating course. It occurs in hyperactive, hypoactive, or mixed forms, in up to 50% of elderly hospital inpatients, many with pre-existing dementia, and appears to be independently associated with significant increases in functional disability, length of hospital stay, rates of admission to long-term care institutions, rates of death, and healthcare costs. Despite its clinical importance, delirium is often not detected or it is misdiagnosed as dementia or other psychiatric illness even though there are potential strategies (e.g., screening by nurses, risk-factor assessment) and instruments that can improve detection and diagnosis. Although there has been limited progress in understanding the etiology, pathogenesis, assessment, and specific treatment of delirium, systematic detection and treatment programs appear to be beneficial for elderly surgical patients, as are preventive programs for elderly medical and surgical patients. Even now, there is probably enough evidence to recommend implementation of these two types of programs in acute-care hospitals.