The potential long-term effects of Traumatic Brain Injury (TBI) are poorly understood. Repeated concussions have been associated with an elevated incidence of Alzheimer's disease (AD) along with a reduced age of onset. As repetitive TBI has been studied, a syndrome has now been identified: chronic traumatic encephalopathy (CTE). There are growing concerns about the long-term neurologic consequences of head impact exposure from routine participation in contact sports (e.g., boxing, football). Brain autopsies of athletes with confirmed CTE have demonstrated tau-immunoreactive neurofibrillary tangles and neuropil threads (known as tauopathy). The relationship between exposure to repetitive head impact and the subsequent development of chronic neurodegenerative disease has not been established. Further, as the diagnosis of CTE (defined by the presence of tauopathy) is presently made at autopsy, clinical tools and biomarkers for detecting it remain to be defined. With the advent of FDA-approved PET amyloid imaging, clinicians and researchers are now able to estimate plaque density in the brains of living patients. However, there are critical limitations to amyloid imaging. Current evidence suggests that markers of the presence and severity of tauopathy may be able to address these limitations. Our study will utilize both [18F] Florbetapir and [18F]-T807 PET imaging to investigate amyloid and tau accumulation in subjects with a history of concussions. We will obtain MRI, PET, and neurocognitive data in a cohort of 25 subjects with a history of TBIs and a cohort of 25 subjects with mild cognitive impairment.We aim to determine whether these individuals are on the same trajectory of neurodegenerative disease seen in AD or in CTE. Because of the overlap in clinical/cognitive and some behavioral symptoms in AD and CTE, an additional biomarker tool is needed to prevent misdiagnosis. Accurate diagnosis is crucial in order to provide patients with appropriate treatment.