Additional Study Information: The Longitudinal Early-onset Alzheimer's Disease Study (LEADS) is a natural history, non-treatment study designed to look at disease progression in individuals with early onset cognitive impairment . Clinical/cognitive, imaging, biomarker, and genetic characteristics will be assessed across two cohorts: early onset Alzheimer's Disease (EOAD) participants and cognitively normal (CN) control participants. Early-onset Alzheimer's disease (EOAD) affects about 5% of individuals with Alzheimer's disease (AD) (~200,000-300,000 cases in the United States). The Longitudinal Early-Onset Alzheimer's Disease Study (LEADS) explores the development of EOAD and how it compares to the more common late-onset form of AD. To successfully move toward effective treatments, researchers need to understand the genetic, biological and clinical processes involved in EOAD. This study involves memory testing, blood and CSF collection, MRI and PET scans. Your participation in LEADS could contribute to future medical advances to help all those affected by AD.