Memory loss and other cognitive dysfunctions, although common in elderly persons, are not universal features of old age. Instead they herald the presence of various neuropsychiatric diseases, which are first recognized as syndromes. The two most common neuropsychiatric syndromes, dementia and delirium, produce global changes in cognition and other capacities. They are differentiated by the patient's level of consciousness, which is impaired in delirium but intact in dementia. Delirium is generally reversible and often indicates serious physical illness. Although dementia is occasionally reversible, the mainstays of its management and treatment are palliative. Comorbid psychiatric symptoms are common in patients with both delirium or dementia, and their recognition and treatment constitute an important task for the geropsychiatrist. The differential diagnosis of primary dementing illness and other psychiatric illnesses such as depression is complex, because symptoms of the two kinds of disorders often coexist and common pathogenetic mechanisms may underlie both syndromes.