The Adult Changes in Thought (ACT) study is a longitudinal prospective cohort study that began in 1994. Participants are randomly selected from Seattle area members of Group Health aged 65 years or older. Participants are cognitively intact at study enrollment, defined operationally as a CASI (Cognitive Abilities Screening Instrument) score of >85 or consensus diagnosis of “not demented” following comprehensive neurological and neuropsychological assessment. Incident cases of dementia are identified with the same 2-stage sampling scheme, where all participants with CASI scores <86 are evaluated with a comprehensive neurological and neuropsychological assessment, and all results are considered at a consensus conference. Autopsy is also available for this collection; autopsy consent rates are about 25% of the cohort. Genomic DNA from blood and/or brain tissue is available from this collection. This study includes 4690 subjects across three sub-cohorts.