AbstractDT plasma operations took place at Joint European Torus (JET) from May to November 1997. The site tritium inventory was 20 g, and this was recycled several times using the active gas handling system (AGHS) to recover tritium from mixed exhaust gases. The amount of tritium available for use at any time was determined by the hold-up in the in-vessel components, in-process inventory, and losses via permeation and transfer to waste. It was therefore necessary, to continually assess the tritium inventory through on-line and batch measurements. During the shutdown periods, tritium continued to be evolved from the machine and its trapped volumes. This meant that a large number of specific measurements were necessary to ensure that the safety requirements were met and that the waste could be properly characterised. During DD operations, the exhaust gas from the machine contained quantities of tritium, too great to be discharged to atmosphere and must continue to be processed by the AGHS. The techniques for tritium inventory assessment are reviewed, taking into account continuing uncertainties and plans for resolutions of these. The JET experiment is used to highlight issues of tritium inventory control, which will apply to future fusion reactors.