*In collaboration with the ITPA Diagnostic Topical Group and the ITER Participating Teams

Abstract. In order to meet the expected needs for first wall and plasma measurements, ITER will require about 40 different diagnostic systems drawn from all the main generic diagnostic groups – magnetics, neutron systems, optical and microwave systems, spectroscopic, bolometric, probes, pressure gauges and gas analysers. The design and implementation of the diagnostic systems is a major challenge because of the harsh environment in which many of the diagnostic components are located coupled with the restricted access and the need to meet stringent engineering requirements arising from the fact that ITER will be a nuclear device. It has stimulated an extensive design and R&D programme and the development of some novel approaches to diagnostic installation: for example, the use of plugs with custom modules at the upper and equatorial levels that serve both to support the diagnostic components and to provide the necessary shielding of the neutrons. In this paper, the difficulties of implementation are summarized and the novel solutions described. An assessment of the performance of the diagnostic system relative to the specified target measurement requirements is given. 1