Purpose. This Funding Opportunity Announcement (FOA) invites research grant 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. Mechanism of Support. This FOA will use the NIH Exploratory/Developmental (R21) award mechanism and runs in parallel with FOAs of identical scientific scope, PA-10-024, that encourages applications under the NIH Exploratory/Developmental Phased Innovation (R21/R33) award mechanism and PA-08-137 that seeks applications using the Small Business Innovation Research (SBIR [R43/R44]) award mechanism. Funds Available and Anticipated Number of Awards. Awards issued under this FOA are contingent upon the availability of funds and the submission of a sufficient number of meritorious applications.