Present ITER diagnostics are designed to provide machine protection, basic and advanced control, fusion performance evaluation, and an extensive measurement capability for furthering plasma physics understanding. However, in the longer term beyond ITER, diagnostic components and associated materials must survive extended periods in the more hostile environment of not only DEMO, but also fusion power plants. In addition to the need to minimize penetrations in the first wall, undoubtedly due to their known high sensitivity to radiation, the use of insulators, and hence diagnostics, will be further severely restricted to those essential to operation, safety, and maintenance related to plasma control and machine protection. The problems we will have to address are related to long-term fluence or dose-related degradation of the required properties due to aggregation and segregation of radiation-induced defects and impurities present in the original materials, as well as H, He, and other transmutation elements. To resolve these challenges, long-term research activities must increase. For the diagnostics (and other systems), in situ irradiation testing is essential. In the near- to mid-term future, available experimental fission reactors will be invaluable, where even basic problems such as irradiation in vacuum and temperature control must be overcome.