Because of the current progress in molecular medicine (genomics, proteomics), a plethora of new and often human-specific targets are being identified. These targets often play a significant role in the pathogenesis of diseases, and identifying them offers the potential for early diagnosis and intervention. An early in vivo validation of specific ligands binding to these targets in humans is needed to as- sess their potential for targeted imaging and radiotherapy. Further- VI Preface more, such validation studies may allow for a better understanding of the molecular processes underlying phannacologic activity and therefore for a more successful development of phannaceuticals in general. The purpose of the Ernst Schering Research Foundation (ESRF) Workshop 48 was to provide a forum for an open exchange on the state of the art in the early development of such radiophanna- ceuticals. Experts from academia, industry, and regulatory authori- ties were invited to give presentations on aspects covering the identi- fication of targets, preclinical studies on the safety of ligands, as well as their validation in human clinical trials. It was our intention to cover both the opportunities and the challenges that scientists in this field are facing. Radiopharmaceuticals are uniquely suitable for the above-men- tioned target validation studies.