The paper reviews the results of interpretive Fokker-Planck modelling of NBI tritons and DT alphas in JET Trace Tritium Experiments. Also presented are the results of predictive Fokker- Planck modelling of NBI deuterons and DT alphas in ITER plasmas. Further discussed are the requirements of the gamma ray and neutron diagnostics for maximising the information that can be obtained about fast ions confined in tokamak plasmas.