To successfully control tritium in ITER, the reactor must implement an appropriate tritium tracking programme. This paper introduces the need to compartmentalize the different tritium systems of ITER and then assesses current practical and theoretical tritium quantification methods. By applying the most suitable measurement techniques, it is possible to establish probable tritium mass uncertainties which are presently achievable in the various ITER systems. On this basis, a number of R&D activities have been identified aimed at reducing tritium mass uncertainties for specific ITER systems.