JET's recent experimental programme proved that 'burning plasma diagnostics', i.e. neutron, alpha particle, He ash, and fuel mixture measurements, can provide very useful information about crucial physical aspects of great reactor relevance. First of all, several of these diagnostics can improve the diagnostic capability of the ion fluid significantly. During TTE spatially resolved neutron measurements at JET were essential in obtaining the isotopic composition and the transport of the hydrogen isotopes, allowing a direct comparison between the measured transport coefficients and the neoclassical theory. The neutron emission profiles can also give crucial indications for assessing the merits of various heating schemes and their current drive capability. Neutron spectroscopy in its turn provides a clear and direct measurement of the temperature and the velocity distribution of the fuel ions. For example, the dependence of the toroidal velocity from the ion cyclotron radiofrequency heating phasing was clearly seen during TTE. The requirements of accurate neutron measurements are also promoting considerable research in detector technology, in particular in the fields of compact spectrometers and solid state detectors. 'Burning plasma' diagnostics can also strongly contribute to the physics of energetic particles and their interaction with the main plasma. γ-Ray spectroscopy is now an established method to determine the spatial localization, to visualize the trajectories of the alpha particles and the fast deuterons and to obtain estimates of their slowing down. A completely new method to detect the energetic particles, exploiting the line intensity ratio of extreme ultraviolet radiation emitted by suitable extrinsic impurities, is also being pursued. This technique allows investigating the energy range below 600 keV, extremely interesting for the study of wave–particle interactions.