Alzheimer's disease is the major cause of dementia in the elderly and afflicts about four million Americans. This chapter focuses on AD and discusses other dementias to the extent to which they contribute to our understanding of AD. While AD is easily the most prevalent dementing disease, other rarer dementias offer useful comparators. These include dementia with Lewy bodies (DLB), prion disease, Worster drought syndrome (British dementia), and frontal temporal dementia (FTD) with tangles (FTDP-17T). The genetic findings have enabled the creation of transgenic mice that model parts of the disease process. While Alzheimer patients are undoubtedly better treated than they were 15 years ago, the only direct benefit to patients from this gene-based approach to research, to date, has been the availability of genetic testing in the kindreds with amyloid beta precursor protein gene (APP) and presenilin mutations.