DT plasma operations took place at JET from May to November 1997. The site tritium inventory was 20 g and this was recycled several times using this Active Gas Handling System to recover tritium from mixed exhaust gases. The amount of tritium available for use at any time was determined by the hold-up in 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 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 safety requirements were met and that 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.