The current progress in molecular medicine allows the identification of a plethora of new and often human-specific drug targets. An early in vivo validation of specific ligands binding to these targets in humans is needed to assess their potential for targeted imaging and radiotherapy. Radiopharmaceuticals are uniquely suitable for such target validation studies. The purpose of the Ernst Schering Research Foundation Workshop 48 was to offer a forum for an open exchange on the state of the art in the early development of such radiopharmaceuticals. Experts from academia, industry and regulatory authorities provided contributions covering the identification of targets, the necessary preclinical studies on the safety of ligands as well as their validation in human clinical trials.