The origin and markers of progression for Alzheimer's disease (AD) are relatively obscure. Despite increased understanding of the underlying biology of AD, its clinical diagnosis is still made only after progressive cognitive decline; definitive diagnosis is confirmed at autopsy. This study will examine biomarker changes over time in a distinct cohort of people with an increased risk of developing AD. The study will also identify and track biological changes that occur with progressive dementia and compare those changes to the known cognitive and emotional disturbances that characterize AD.