Additional Study Information: This study is associated with the Alzheimer's Disease Research Center (ADRC) at Columbia University. The study seeks to understand changes in the way the brain's structure and function are related to age, and the risk of developing memory problems, Alzheimer's disease (AD), and dementia. The study will use magnetic resonance imaging (MRI) to examine these functional and structure measures, obtain cerebrospinal fluid (CSF) by performing a lumbar puncture (LP) to detect any biological markers of Alzheimer's disease, and conduct cognitive testing that measures different kinds of thinking abilities. You will be compensated a total of $250 for your participation. The MRI includes a contrasting agent called gadolinium (Dotarem) that is FDA-approved. The lumbar puncture is a standard clinical procedure and is performed by Dr. Honig, who is the director of the clinical core at the ADRC and a highly experienced neurologist with 30+ years of experience. The study aims to determine how brain measurements, any presence of biomarkers and performance on cognitive tasks can help distinguish normal cognitive aging from cognitive changes due to Alzheimer's disease. All parts of the study are scheduled over three visits that are as close to each other as possible (but no more than 6 months apart). All testing is done at the Columbia University Medical Center in Washington Heights (W 168th St.). Participants must be 65 years or older and have no previous history of gadolinium allergy, kidney disease, severe asthma or COPD, ferromagnetic implants, chemotherapy or radiation, or blood thinners. Further exclusion criteria apply and will be discussed during initial phone contact.