Organic mental disorders with cognitive impairment constitute an important medical and public health problem in elderly people. The emergence of severe acute psychiatric manifestations of delirium as a consequence of structural brain disease or cerebral dysfunction related to some other physical illness is well known in clinical practice. After a long period of nosological inconsistency, the DSM-111, and now the DSM-111-R, present operational diagnostic criteria for delirium emphasizing the cognitive symptoms of the disorder. At the same time different psychomotor symptoms in, for example, hyperkinetic, mixed, or silent delirium are also being recognized. In addition, many clinical studies have illustrated the wide variation in predisposing, precipitating, and sustaining factors for delirium, thus providing a basis for etiological hypotheses.