Since the first operation of JET in 1983, there has been a considerable program of diagnostic development to keep pace with the requirements of the experimental program. Many diagnostic systems have been upgraded and several new ones have been added. New diagnostics are being built and others modified for the pumped divertor which will come into operation at the end of 1993. A few months ago JET carried out a preliminary experiment with tritium which produced the first significant fusion power (over 1.5MW) and provided the first real test of diagnostics in a high radiation environment. A more extensive experimental program in tritium is planned for 1995 and 1996.