The concept of dementia should not be used synonymously with the concept of organic mental disorders. By definition, according to DSM-III and ICD-10, dementia is a syndrome which includes memory impairment. The severity of the disorder is disabling and the course is chronic. Differential diagnosis includes age-associated memory impairment (AAMI), delirium, and depressive disorders. The dementias may be subdivided into four groups: idiopathic (primary degenerative dementias), vascular, secondary, and others. The idiopathic dementias are those in which etiology is assumed to be found within the brain itself. The main subgroup is Alzheimer-type dementia. The vascular dementias are those in which the blood supply to the brain is insufficient. Multi-infarct dementia (MID) is the prototype. In secondary dementias, somatic disorders either within or external to the brain cause the dementia.