AbstractThe most common errors in the diagnosis of dementia are underdiagnosis and overdiagnosis. Underdiagnosis is best avoided by careful use of psychometric screening tests to detect cognitive impairment in patients unaware of their own defect. Overdiagnosis can be avoided by not applying the label of “dementia” until the diagnosis is unequivocal. Complete evaluation is necessary to rule out treatable causes of dementia, and should include medical, neurological, and psychiatric history and examination, simple psychometric tests, laboratory battery, computerized transaxial tomographic brain scan, and electroencephalogram. Ultimately, the diagnosis of dementia may change at follow‐up examination, and the unequivocal diagnosis of a specific form of dementia still requires autopsy and neuropathological examination of the brain. The most common causes of cognitive impairment in the elderly include Alzheimer's disease, multi‐infarct dementia, depression, and other specific illnesses such as tumors, Jakob‐Creutzfeldt disease, and normal pressure hydrocephalus. Even systemic disease can present as dementia. The evaluation outlined above may help distinguish among these causes. New diagnostic techniques may eventually be helpfu