This study investigates the impact of aspects of tax design on the operating costs of the tax system. The thesis focuses on the Australian and UK regimes for taxing the capital gains of individuals. It contends that the compliance burden faced by personal taxpayers and the administrative costs incurred by revenue authorities are directly influenced by the design of the capital gains tax ('CGT') regimes in each country. The study bridges the divide between theoretical analysis of CGT and empirical studies on tax operating costs. It uses a hybrid research design to test a series of hypotheses that emerge from a review of the literature and the experience of the researcher. It combines a technical analysis of the relevant Australian and UK legislative provisions (including an analysis of the policy and other background data that underpins those provisions) with empirical research on the views and experience of practitioners who are responsible for the operation of the legislation in the two countries. The results obtained from this combined methodology indicate that the operating costs of taxing capital gains in Australia and the UK are directly affected by the design of the legislative provisions. Moreover, the study outcomes indicate that operating costs in both countries are high (on a number of comparative measures), have not reduced over time, and are both horizontally and vertically inequitable. The research indicates that the primary factors that cause the high operating costs include the complexity of the legislation and the frequency of legislative change, together with record-keeping and valuation requirements. The thesis identifies specific legislative changes that would address operational cost concerns. These include the phasing out of the 'grandfathering' exemption together with the introduction of an annual exempt amount, and the rationalisation of business concessions in Australia; and the abolition of taper relief and its possible replacement with a 50% exclusion in the UK. More importantly, it seeks a more principled approach to the taxation of capital gains in both countries, and emphasises that legislative change can and should only be enacted with a full and clear understanding of the operating cost implications of that change.