An epidemiology closely linked with the increase in life span. In most countries, the prevalence of dementia varies between 6 and 8% for individuals aged 65 years or more. It then dramatically increases with each subsequent decade, reaching around 30% of the population aged over 85. The costs associated with dementia are correlated with the increase in age and are of increasing concern for politicians, healthcare professionals and family members of demented patients. Current estimations are approximate, but dementia appears to be the most costly disease for society after the age of 65 years in France, the Netherlands, Sweden, or the United States. Detailed cost analyses have distinguished the direct medical,direct non-medical and intangible costs. The most important contribution in costs for society is the long-term care by health care professionals (institutionalization corresponding to 2/3 of the total costs for society!), but the care provided by the helpers and the families is even greater, even though difficult to quantify. The current question is to know whether present and future medical treatments will be able to reduce the tremendous financial costs of this chronic disease.