Inclusion Criteria:

Former NFL Players

• English as primary language.
• No MRI or Lumber Puncture (LP) contraindications.
• Have played ≥ 12 years of organized football (including ≥ 3 in college and ≥ 3 seasons in the NFL).
• Must have played one of following positions offensive lineman, defensive lineman, linebacker, tight end, wide receiver, running back, or defensive back.
• Agree to provide name and contact information of a study partner who will complete questionnaires regarding subject's mood, behavior, thinking and cognitive function should the subject enroll in the study.

Former Collegiate Football Players

• English as primary language.
• No MRI or Lumber Puncture (LP) contraindications.
• Must have played ≥ 6 years of organized football (with ≥ 3 years at the college level, but no organized football or other contact sport following college).
• Must have played one of following positions offensive lineman, defensive lineman, linebacker, tight end, wide receiver, running back, or defensive back.
• No military service.
• Agree to provide name and contact information of a study partner who will complete questionnaires regarding subject's mood, behavior, thinking and cognitive function should the subject enroll in the study.

Control Group

• English as primary language.
• No MRI or Lumber Puncture (LP) contraindications.
• No history of organized contact sports or known traumatic brain injury (TBI) and/or moderate/severe concussions.
• No history of participation in any of the following organized sports at any level: football, hockey, rugby, soccer, lacrosse, wrestling, boxing, gymnastics, martial arts, and kickboxing.
• No military service.
• Must have BMI ≥ 24.
• Agree to provide name and contact information of a study partner who will complete questionnaires regarding subject's mood, behavior, thinking and cognitive function should the subject enroll in the study.
• Must be asymptomatic when screened by telephone.

Exclusion Criteria:

Former NFL and Varsity Collegiate Football Players

• If they do not meet inclusion criteria.
• If they have a history of clinical stroke confirmed on neuroimaging.
• If they have vision or hearing impairment significant enough to compromise neuropsychological testing.
• If they are unable to undergo MRI/PET Scan.
• If they are unable to travel to one of 4 study sites to participate.
• If they cannot provide the name and contact information of an eligible study partner.
• If they are taking blood thinners that would make LP unsafe.
• If they do not agree to all study tests and procedures.
• If they are unable to consent to study procedures.
• If they have been hospitalized or treated in an emergency room following a severe injury to their head since they stopped playing football (severity determined on phone screening).

Control Group Exclusion:

• If they do not meet inclusion criteria.
• If they have vision or hearing impairment significant enough to compromise neuropsychological testing.
• If they are unable to undergo MRI/PET Scan.
• If they are unable to travel to one of 4 study sites to participate.
• If they cannot provide the name and contact information of an eligible study partner.
• If they report any cognitive concerns or dementia during screening process.
• If they are unable to consent to study procedures (during eligibility screening and the written consent for study procedures).