Optimisation of auxiliary plasma heating by means of Ion Cyclotron Radio-Frequency (ICRF) and Neutral Beam Injection (NBI) as envisaged for the future fusion reactors is one of the main priorities in present research at today’s tokamaks. Therefore, investigation of the production of fast ions during heating and of the subsequent fast ion behaviour in magnetically confined plasmas, together with an evaluation of the resulting bulk ion heating efficiency, are of essential importance for fusion reactor development. Gamma-ray diagnostics, based on the measurement of the gamma-ray emission from nuclear reactions between fast ions and the main plasma impurities, is a valuable technique for studying the fast particle energy distributions. Gamma-ray spectrometry provides information on the energy distribution, and the measurement of emission profiles supplies information on the spatial distribution of the reaction sites. Since 1987, the γ-ray emission from JET plasmas has been systematically monitored and used successfully in the analysis of heating effects during ICRF and NBI heating in the JET tokamak. The classical character of the fast ion slowing down behaviour has been demonstrated and estimates have been obtained of the fast particle confinement time [1]. The study of sawtooth crashes has demonstrated dramatic spatial redistribution of fast particles and other effects [2]. In recent JET experiments to study the ITER-relevant ICRH scenarios (3He)D and (3He)4He, γ-ray measurements provided information on the fast ion population, with the effective temperature of the energetic tail ions being deduced with the help of a γ-ray spectrum simulation code, GAMMOD. In this paper, the main γ-ray results are presented and the capabilities of gamma diagnostics are discussed in the light of the ITER-project programme.