The JET Neutral Beam Injection (NBI) system has proved to be an extremely effective and flexible heating method capable of producing high performance plasmas and performing a wide range of related physics experiments. High fusion performance deuterium plasmas have been obtained in the Hot lon (HI) H-mode regime, using the central particle fuelling and ion heating capabilities of the NBI system in low target density plasmas, and in the Pellet Enhanced Plasma (PEP) H-mode regime, where the good central confinement properties of pellet fuelled plasmas are exploited by additional heating and fuelling as well as the transition to H-mode. The HI H-mode configuration was used for the First Tritium Experiment (FTE) in JET in which NBI was used to heat the plasma using 14 D0 beams and, for the first time, to inject T0 using the two remaining beams. These plasmas had a peak fusion power of 1.7MW from D-T fusion reactions. The capability for injection of a variety of beam species (H0, D0, 3He0 and 4He0) has allowed the study of confinement variation with atomic mass and the simulation of a-particle transport. Additionally, the use of the NBI system has permitted an investigation of the plasma behaviour near the toroidal limit over a wide range of toroidal field strengths.