Alzheimer's Disease Neuroimaging Initiative 3 (ADNI3) is a study aiming to understand Alzheimer's Disease (AD), a brain illness that affects memory and thinking. The study involves different groups of people, some with normal memory, some with mild memory issues, and others with Alzheimer's. Over 5 years, participants will have various tests, like memory check-ups and brain scans (MRI and PET scans). These tests help researchers understand how AD progresses from normal aging to more severe memory problems. **Key Points:** - The study is a non-treatment study, meaning participants won't receive any specific treatment for AD during the study. - It involves multiple visits and tests over 5 years, including brain scans and blood tests. - Participants may have some risks, like discomfort from blood tests or lumbar punctures (a test where fluid from the spine is collected). Eligibility: Participants must be 55-90 years old, speak English or Spanish, and have a study partner to attend visits with them. Participants should not have serious health issues that could interfere with the study and must be willing to undergo several brain scans and tests.