The investigators will conduct a tau PET scan in cognitively normal older adults and patients with Mild Cognitive Impairment (MCI), enrolled in the National Alzheimer's Coordinating Center (NACC) study at the University of Pennsylvania's Penn Memory Center/Alzheimer's Disease Core Center (PMC/ADC).

This is a cross-sectional and longitudinal study using the radiotracer [18F]-PI-2620 to determine the relationship of tau pathology to both cross-sectional and longitudinal clinical and biomarker data of NACC cohort participants who are Cognitively Normal or MCI. All subjects will already be part of the longitudinal cohort study, known as the "NACC" cohort, of the PMC/ADC (protocol 068200). Participants will provide informed consent for this protocol before beginning any study procedures. After screening assessments, participants will undergo PET scan imaging with [18F]PI-2620. There will be one follow-up [18F]PI-2620 PET scan approximately 18 +/- 6 months after bas eline scan.

Inclusion Criteria:

• Males and females ≥ 60 years of age.
• Part of the NACC longitudinal cohort (IRB# 068200) of the PMC/ADCC with consensus conference designation of cognitively normal or Mild Cognitive Impairment (MCI) or Alzheimer's Disease (AD) Possible or Probable.
• NACC longitudinal visit completed or scheduled within 6 months before or after enrollment in this study.
• Women must be post-menopausal or surgically sterile.
• An amyloid PET scan, completed as a part of the NACC protocol (IRB# 825943), must be performed or scheduled within 12 months before or after the [18F]PI-2620 PET scan.
• A brain MRI, completed as a part of NACC protocol (IRB# 068200), must be performed or scheduled within 6 months before or after the [18F]PI-2620 PET scan. Scan should be of adequate research quality, including 3 Tesla and/or 7 Tesla high-resolution imaging of MTL structure.

Exclusion Criteria:

• Have any medical or psychiatric conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study.
• Have evidence of structural abnormalities such as major stroke or mass on MRI that is likely to interfere with analysis of the PET scan.
• Inability to tolerate or contraindication to imaging procedures in the opinion of an investigator or treating physician.
• Have a history of significant or ongoing alcohol abuse or substance abuse or dependence based on medical record review or self-reported.

The inclusion / exclusion criteria will be ascertained through self-report in conjunction with any medical history available through the participant's medical or research records (EPIC or the ADC database)

Adults cognitively normal or Mild Cognitive Impairment (MCI) or Alzheimer's Disease.