Alzheimer's disease and cerebrovascular disease are the most common causes of dementia. However, many other neurological and general medical conditions produce this syndrome as well. In a review of 32 studies of dementia collectively including 2,889 subjects, Alzheimer's disease was the diagnosis in 56.8% of cases, vascular dementia in 13.3%, and depression in 4.5%; other medical and neurological causes such as those reviewed in this chapter accounted for approximately 16.7% of cases (Clarfield 1988). These frequencies are, however, neither definitive nor accepted universally. Among persons older than 65 years, there is general agreement that Alzheimer's disease is the most common cause of dementia. There is considerable disagreement regarding the relative frequencies of causes of dementia other than Alzheimer's disease, with some studies suggesting vascular dementia and other studies suggesting dementia with Lewy bodies as the second most common cause of dementia in this age group (McKeith et al. 2004).