This Funding Opportunity Announcement (FOA) issued by the National Institute of Mental Health (NIMH), the National Institute on Aging (NIA), the National Institute of Alcohol Abuse and Alcoholism (NIAAA), the National Institute of Biomedical Imaging and Bioengineering (NIBIB), and the National Institute on Drug Abuse (NIDA) of the National Institutes of Health (NIH), invites NIH Exploratory/Developmental Grant (R21) applications from organizations/institutions that propose the development of novel radioligands for positron emission tomography (PET) or single photon emission computed tomography (SPECT) imaging in human brain, and that incorporate pilot or clinical feasibility evaluation in pre-clinical studies, model development, or clinical studies.