Inclusion criteria:  Age 50 and older at time of study entry. Meet criteria for either a) amnestic mild cognitive impairment (single or mixed domain) or Alzheimer's disease, or b) have no cognitive impairment, based on history, exam, and neuropsychological testing. Patients must have Clinical Dementia Rating Scale score of 0.5 or 1 at enrollment. Controls must have Clinical Dementia Rating scale score of 0 at enrollment. Subjects must have AD biomarker previously obtained for research or clinical purposes or undergo a 18F-florbetaben PET scan during the screening process. Patients must have positive amyloid PET scan or CSF results consistent with AD. Controls must have a negative amyloid PET scan or CSF results not consistent with AD. Self-identify as white, non-Hispanic or Latino Subjects must be ableto provide informed consent Written and oral fluency in English Able to participate in all scheduled evaluations and to complete all required tests and procedures. In the opinion of the investigator, the subject must be considered likely to comply with the study protocol and to have a high probability of completing the study.  Exclusion Criteria:  Past or present history of certain brain disorders other than MCI or AD. Certain significant medical conditions, which make study procedures of the current study unsafe. Such serious medical conditions include uncontrolled epilepsy and multiple serious injuries. Contraindication to MRI scanning Conditions precluding entry into the scanners (e.g. morbid obesity, claustrophobia, etc.). Exposure to research related radiation in the past year that, when combined with this study, would place subjects above the allowable limits. Participation in the last year in a clinical trial for a disease modifying drug for AD. Inability to have a catheter in subject's vein for the injection of radioligand. Inability to have blood drawn from subject's veins. Taking anticoagulant (e.g., warfarin) or immunosuppressive/immunomodulatory medication. Nonsteroidal anti-inflammatory drugs (NSAIDs) are not exclusionary. Use of steroids in the 30 days preceding the PET scan. Having a diagnosis of a chronic inflammatory disease (for example, multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis, psoriasis, systemic lupus erythematosus) or a chronic infectious disease such as H.I.V.