This book is a compilation of the invited papers, which were presented at the Fourth European Symposium on Radiopharmacy and Radiopharmaceuticals, which was held in Baden, Switz- land, 1-4 May, 1991. The First and Third Symposia on Radiopharmacy and Radiopharmaceuticals (Elsinore, Denmark, 1983, 1987) concentrated on the safety and efficacy of radiopharmaceuticals, whereas this Fourth Symposium to some extent followed up the subject of the Second Symposium (Cambridge, UK, 1985): recent developments in radiopharmacy and current research on radiopharmaceuticals. The symposium was organized by the Radiopharmacy Group of the Swiss Society of Medical Radiology (Section Nuclear Medicine) under the auspices of the task group on radiopharmaceu- cals of the European Association of Nuclear Medicine (EANM). The organizing committee consisted of the cochairmen Drs. P.A. Schubiger (Paul Scherrer Institute (PSI), Villigen) and G. Westera (University Hospital, ZiJrich) and the members H.-F. Beer, P. Bl~iuenstein, P. Hasler (all PSI) and H. M~icke (Cantonal Hospital, Basel). The subjects of this Symposium ranged from isotope production to clinical testing of radioph- maceuticals, including the organisational prerequisites. In addition, the development of new radiopharmaceuticals and of PET radiopharmacy, and the concomittant ongoing evolution of regulatory guidelines by national (various European countries, USA) and international (EC) authorities, induced us to honor the vivid interest in this subject and to make it an important part of this symposium.