The National Institute on Drug Abuse (NIDA), the National Institute on Aging, the National Institute of Neurological Disorders and Stroke (NINDS), and the National Institute of Mental Health (NIMH), through the issuance of this Request for Applications (RFA), seeks to encourage broader uses of established PET/SPECT radioligands by reducing barriers to their wider distribution, and expanding their utility to the study of diseases or organs for which the radioligand has not previously been studied. Applications for this RFA should demonstrate a high degree of innovation and novelty with regard to the new uses for existing radioligands. Although there is no requirement for preliminary data, a clear scientific rationale is essential. Applications for this RFA are expected to propose multi-institutional collaborations between investigators who have the capacity for routine production of a given radioligand for human use, and investigators who lack access to the radioligand but wish to demonstrate the feasibility of an innovative use for the radioligand in a novel patient population. The primary focus of the proposal must be on human studies; animal studies are allowable only if required to obtain regulatory approval for the ligands.