Ten to 33 percent of the patients who present for evaluation of dementia are found to have a potentially reversible cause of dementia such as metabolic, structural, or psychiatric condition. Another group of patients who present with symptoms or complaints of impaired thinking are erroneously diagnosed as suffering from dementia. The author reviews the criteria for diagnosis of dementia, the laboratory and other tests advisable for use in evaluating the presence of dementia or guiding the treatment of irreversible dementia, and some factors that may lead to the misdiagnosis of dementia. He emphasizes that a thorough medical, psychiatric, and psychosocial evaluation of all patients presenting with cognitive impairments is essential.