The Mayo Clinic Study of Aging was designed to establish a prospective population-based cohort of subjects to study prevalence, incidence and risk factors for MCI and dementia. The study was conducted in Olmsted County, where several factors enhance the feasibility of population-based epidemiologic research. Most residents seek care within the community from essentially 2 providers, the Mayo Clinic and Olmsted Medical Center, along with their affiliated hospitals and medical facilities within the county. Both healthcare providers use a unit medical record which includes all outpatient and inpatient information for each patient. The study began October 1, 2004, and recruitment is ongoing to maintain a sample size of about 3,000 active participants. Participants are randomly selected from the Olmsted County population using a sex and age-stratified sampling scheme. Selected participants are invited to participate; exclusion criteria are persons who are in hospice or terminally ill or demented, but recruitment of persons with dementia began in 2015. Nearly 6,000 participants have been recruited to date. At the onset of the study, only 70-89 year olds were eligible. Recruitment of 50-69 year-olds began in 2012, and recruitment of 30-49 year olds began in 2014. Followup is performed every 15 months for 50 yrs and older; every 30 months if younger than 50 years by face-to-face visits in the Center, in-home, or by phone for a minority who decline either of the two but still would like to participate. Medical records are reviewed is used to identify prevalent and incident medical conditions (e.g. vascular diseases such as diabetes, hypertension) and incident dementia cases among persons lost to follow-up.