The understanding of fast ion physics is important for modelling and interpretation of Neutral Beam Injection (NBI) in tokamaks. It is required for the derivation of transport coefficients and the simulation of heating and current drive in beam heated plasmas. The JET tokamak possesses a unique combination of tools to diagnose NBI fast ion behaviour in the ability to inject on- and off-axis tritium beams and to diagnose them using a 2-D neutron camera.