A Study of Flortaucipir PET in Healthy Volunteers and Cognitively Impaired Subjects

Inclusion Criteria: * Healthy Volunteers * Male or female subjects ≥ 50 years of age * Mini-mental state examination (MMSE) ≥ 29 * MCI Subjects * Male or female subjects ≥ 50 years of age * MMSE ≥ 24 * Have MCI consistent with National Institute on Aging-Alzheimer's Association (NIA-AA) working group's diagnostic guidelines for AD * Have a study partner that can report on subject's activities of daily living * Probable AD Subjects * Male or female subjects ≥ 50 years of age * MMSE \> 10 * Meet clinical criteria for probable AD based on the NIA-AA working group's diagnostic guidelines for AD * Have a study partner that can report on subject's activities of daily living Exclusion Criteria: * Current clinically significant psychiatric disease * Evidence of structural brain abnormalities * Evidence of dementing illness other than AD * Current clinically significant cardiovascular disease or ECG abnormalities, or additional risk factors for Torsades de Pointes * Current clinically significant infectious disease, endocrine or metabolic disease, pulmonary, renal or hepatic impairment, or cancer * History of alcohol or substance abuse or dependence * Females of childbearing potential who are not surgically sterile, not refraining from sexual activity or not using reliable methods of contraception * Have received or participated in a trial with investigational medications in the past 30 days * have had a non-study related radiopharmaceutical imaging or treatment procedure within 7 days prior to the study imaging session.