JET has an extensive range of plasma diagnostics systems that have been designed and constructed to be compatible with the requirements of a large fusion experiment that routinely produces plasmas with parameters in the range required for a fusion reactor. Many of these diagnostics are designed to continue working when tritium is used at a later phase of the JET programme. Two groups of diagnostics, for profiles and for MHO fluctuations, are reviewed in more detail and some typical results are shown. New diagnostics are being designed for the pumped divertor that will be installed as a major modification of JET as part of the proposed programme extension in the years 1992 to 1996.