Patients who are critically ill often develop a variety of psychiatric symptoms, which require assessment and treatment. The most common psychiatric disorder in the intensive care unit is delirium. Depressed mood and anxiety also occur, at times as discrete disorders, but more often secondary to delirium. Patients with severe mental illnesses, such as schizophrenia and bipolar affective disorder, also may become critically ill-assessment and management of these patients often requires specialized psychiatric care and is not addressed here.