The increasing and widespread interest in age-associated dementias originated because of the rapid aging of the world population. Considering the rapid demographic changes and current epidemiologic data indicating a steep increase in the frequency of dementia with advancing age, several authors have introduced the concept of an "epidemic of dementia". Epidemiological research gives the opportunity to provide both scientific data and background information for health policy makers. Information concerning the frequency of dementias, their geographical distribution, as well as related disability, is an essential requirement for cost-effective planning of health care services. At the same time, data from population-based surveys, and in particular from cross-national studies, may give clues to the etiology, diagnosis and treatment of the dementias.