Delirium, dementia, and amnestic and other cognitive disorders are classified as cognitive disorders in DSM-IV-TR (American Psychiatric Association 2000). As a group, they represent psychiatric disturbances formerly described as exclusively due to "organic" as opposed to "functional" etiological factors. As research into the etiology and treatment of other psychiatric disorders has progressed, the artificial distinction between organic (an anachronistic term in current clinical practice) and functional psychiatric illness has blurred substantially. Nonetheless, these cognitive disorders generally have clear structural and functional disturbances in brain function as their primary causes. Psychological factors are still very relevant in the patient's experience of symptoms and his or her behavioral and emotional response to illness. Delirium, dementia, and the other cognitive disorders make clear the need for psychiatric evaluation based on the biopsychosocial model of psychiatric illness.