The physicsissues of the measurements of the plasma properties necessary to provide both thecontrol and science data for achieving the goals of the ITER device are discussed. Theassessment of the requirements for these measurements is first discussed, togetherwith priorities that relate to the experimental program. Subsequently, some of theproposed measurement techniques, the plasma diagnostics, are described with particularemphasis on their implementation on ITER and their capability to meet therequirements. A judgement on the present status of the diagnostic program on ITER isprovided with some indication of the research and development program necessary todemonstrate viability of techniques or their implementation.