The main objective of this study is to use a novel PET approach to measure tau accumulation in the brain. The presence of CTE at autopsy in deceased National Football League (NFL) players has been well documented. Accordingly, we will conduct this study in a group of retired NFL players who have clinical symptoms of CTE and are suspected of having CTE based on high levels of tau in their spinal fluid and abnormalities seen on research brain scans. We will compare them with a control group of former elite level athletes who have not experienced any brain trauma, deny any clinical symptoms, and who have completely normal spinal fluid tau and amyloid levels, and brain scans. We will also include a group of subjects with AD. All participants will be recruited from ongoing studies, headed by the Partnering PI of this proposal, Dr. Robert Stern, at the Boston University Center for the Study of Traumatic Encephalopathy and the Alzheimer's Disease Center. We will use both a beta amyloid PET scan ([18F]-florbetapir) and a tau PET scan ([18F]-T807) on consecutive days. With the beta amyloid scan we expect little or no evidence of amyloid in the NFL players with presumed CTE, and no evidence of amyloid in the control group of athletes with no history of repetitive brain trauma. In contrast we expect to see beta amyloid accumulation in the AD patient brains. With the new tau ligand, we expect that the NFL players with presumed CTE will show elevated levels of tau protein in the brain, which will not be observed in athletes without a history of brain trauma, but which will be seen in the AD patients' brains.