Studies establishing key phenomena and developing diagnostics for energetic particle physics, which are essential for the next step burning plasma experiments such as the International Thermonuclear Experimental Reactor (ITER), have been performed at the Joint European Torus (JET). Experiments have demonstrated clear self-heating of deuterium-tritium (D-T) plasma by alpha particles as a maximum in electron temperature at an optimum mixture of 60 ±20% tritium. The change in electron temperature produced by alpha heating, ΔTe(0) = 1.3 ± 0.23 keV, was as expected from classical heating, whereas the heating of thermal ions was higher than expected from reference deuterium discharges. Alfvén eigenmodes were stable in the highest fusion performance D-T plasmas, in agreement with the modeling. Systematic studies on the existence and properties of Alfvén eigenmodes with external antenna driving and detecting Alfvén eigenmodes are presented. The formation of fuel ion tails due to alpha-particle knock-on effects is described as derived from neutral particle analyzer and neutron emission spectrometry in D-T experiments. The gamma-ray diagnostics are shown to measure profiles and energy distribution functions of high-energy ions and alpha particles. Time- and space-resolved gamma-ray images demonstrated for the first time the possibility of measuring several types of energetic ions simultaneously. The novel technique of detecting unstable Alfvén eigenmodes with interferometry is found to be superior in detecting core-localized Alfvén eigenmodes.