Diagnostic advances and their impact on our understanding of Tokamak relaxation phenomena

The recent advances in techniques for diagnosing the internal structure of tokamak plasmas, particularly soft X-ray imaging, electron cyclotron emission and transmission interferometry, are reviewed. These advances have had a major impact on the understanding of tokamak relaxation phenomena, in particular sawtooth oscillations, and this impact is described. Further improvements in diagnostics are in progress and those which are likely to be of benefit in this area of tokamak research are identified.