Design: Patients with signs and symptoms of Alzheimer s disease will be recruited from the community using advertisements and through communication with neurologists, psychiatrists, and general practitioners in the community. Patients will come to the NIH for an evaluation that may include medical and neurological evaluation, laboratory testing, brain MRI, neuropsychological evaluation, and brain imaging with [18F]fluorodeoxyglucose ([18F]FDG) and carbon-11 labeled Pittsburgh Compound B (PIB) PET. The focus of this protocol will be on assisting with the diagnosis of AD vs. another cause of memory impairment. Eligible patients will be offered participation in other protocols with the Molecular Imaging Branch. Eligible patients may also be referred to other studies at the NIH. Patients will not be offered treatment or long-term follow-up in this protocol. Age matched healthy volunteers will undergo brain imaging with MRI and with [18F]FDG and PIB PET. Healthy volunteers and patients who meet diagnostic criteria for AD will be asked to repeat study procedures after an interval of at least one year but no more than 5 years.