Abstract. An extensive diagnostic system will be installed on ITER to provide the measurements necessary to control, evaluate and optimise the plasma performance and to study burning plasma physics. The system will comprise about 40 individual measurement systems drawn from the full range of modern plasma diagnostic techniques. Because of the harsh environment, diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic implementation. In addition, the diagnostic designs have to satisfy stringent engineering requirements. In this paper, we describe the key problems encountered and give examples of the solutions that have been developed. Brief descriptions of the strategies adopted in the selection of the individual diagnostic systems, and in the integration of the systems onto the tokamak, are included. We conclude with an assessment of the overall system performance.