An overview of experimental observations of fast-particle behavior in Joint European Torus (JET) plasmas is presented. The material is drawn directly from the results of measurements based on nuclear detection techniques. The earliest observations concern escaping 15-MeV protons from the D-3He reaction; they are detected in the form of spikes at the time of sawtooth crashes. Subsequent observations with a neutron multicollimator show that sawteeth expel neutral beam injected 80-keV deuterons from the central region of the plasma (but not necessarily out of the plasma). Extensive use has been made of the detection of gamma rays created when ion cyclotron resonance frequency (ICRF)-driven fast ions react with plasma fuel ions and with the main plasma impurity ions carbon, oxygen, and beryllium. Threshold reactions show that ICRF-driven ions can exceed energies of 7.5 MeV. Using ratios of gamma-ray intensities, tail temperatures in the mega-electron-volt range have been diagnosed. The energy content of these ions can exceed 1 MJ and can be as much as one-third of the total energy content of the plasma. Finally, the measurement of 14-MeV neutrons emitted during the burnup of tritons generated by the deuterium-deuterium reaction indicates that the single-particle behavior of 1-MeV tritons is classical within 20%, which implies similar behavior for 3.5-MeV alpha particles in deuterium-tritium plasmas.