The proceedings of a discussion workshop on "Test Phantoms and Optimization in Diagnostic Radiology and Nuclear Medicine" are now available in book form. The workshop, which took place in Wuerzburg, Germany, on 15-17 June 1992, was jointly organized by Radiation Protection Actions (DG XII/F-6 and DG XI/A-1) of the European Commission and GSF-Forschungszentrum fuer Umwelt und Gesundheit in collaboration with the International Commission on Radiation Units and Measurements (ICRU) and the European Federation of Organizations for Medical Physics (EFOMP). It resulted from earlier Commission workshops on diagnostic radiology where the need was recognized to define a common acceptable ground for the evaluation of various test phantoms and objects. In Wuerzburg, some 180 experts from research, medicine and industry discussed possible answers to questions in the following areas: - The technical and physical parameters to be measured for a special clinical field; - Parts of the radiological processes to be assessed by a phantom; - Changes of system performance to be detected and assessed; - Rational use of the results of phantom measurements and assessments; - Characteristics and limitations of available and desirable phantoms; - Optimization of phantom design and performance related to the assessment of the performance of the imaging system, the levels of doses employed and the performance of the human observers; - The impact of phantom measurements on specific clinical measurements; - Elements for a consensus on phantom design, use and evaluation. The discussions, together with 20 overview papers, backed up by 56 poster presentations, are reproduced in the published proceedings. The 400-page publication provides an unique overview and summary of the actual requirements of test phantoms and objects. Subject areas discussed cover a broad range, starting with the role of phantoms in the standardization of the radiological process, passing via clinical requirements to the specific requirements of the technical aspects in the various diagnostic radiological and nuclear medicine processes, e.g. conventional procedures, paediatric radiology, fluoroscopy, CT, mammography, digital radiography, gamma cameras, SPECT and the field of computational models. The workshop coincided with the publication of the ICRU report "Phantoms and computational models in therapy, diagnosis and protection" which is presented in an overview paper. As an additional special initiative, 25 phantoms, presented by posters, are described on standard data sheets.