Paquid (personnes âgées quid) is a population-based cohort specifically designed to study the epidemiology of brain aging and dependency in activities of daily living in elderly people. At baseline screening, 3.777 subjects older than 65 were randomly selected in 75 different parishes from Gironde and Dordogne, and two administrative districts around Bordeaux in South-Western France. The participation rate was 68%. Subjects were followed up every two to three years with repeated measures of cognition, instrumental and basic activities of daily living collected by a trained psychologist, and a systematic detection of incident cases of dementia. The participation rate of each follow-up screening was around 75%. The detection of dementia was conducted with a two-stage design, with a first stage conducted by the psychologist and the confirmation of the diagnosis made at home by a physician, specialist in Alzheimer disease and related disorders (ADRD). Over 20 years of follow-up more than 800 subjects developed incident dementia and more than 2.500 died. Paquid remains the only representative cohort of elderly people living at home in France, giving estimations of prevalence, incidence and duration of ADRD and dependency. Thirty six risk factors of dementia and/or AD have been studied. On the basis of the Paquid data, we have shown that the prodromal phase of AD was longer than ten years and that dementia represented the major cause of dependency in the elderly.