Neutral Beam Injection (NBI) is a very flexible auxiliary heating method for tokamak plasmas, capable of being efficiently coupled to the various plasma configurations required in the Deuterium-Tritium Experimental campaign (DTE1) undertaken in the JET device during 1997. In particular, experiments for high fusion yield and amplification factor Q require intense NBI heating, and for maximum performance and optimum fuel mixture control in D-T plasmas it was necessary to operate the JET NBI systems in both deuterium and tritium. All technical aspects of the modifications to the JET NBI systems for compatibility with tritium operation are discussed, and the associated commissioning is described, including preparatory commissioning using deuterium. Problems experienced and their resolution are highlighted. Some specific beamline physics issues relating to tritium operation are discussed in detail, in particular experimental measurements of beam-target D-T reactions occurring in beam-stopping elements and associated modelling of isotope exchange in these components. Data on NBI performance and tritium usage and recovery for the DTE1 campaign are presented.