In Chapter 1, I laid out the thesis that an international tax regime exists and that it has a coherent structure based on two principles: the single tax principle (that all income should be subject to tax once, not twice or more and not less than once) and the benefits principle (that active income should be taxed primarily at source, whereas passive income should be taxed primarily at residence).

Although the benefits principle is broadly accepted as reflecting the consensus compromise reached under the auspices of the League of Nations in the 1920s, there is a debate on whether there exists an international tax regime (and in particular whether it incorporates a single tax principle). Whether double nontaxation is an appropriate goal of international tax has been hotly debated, for example, in the 2004 Vienna Congress of the International Fiscal Association.

In this concluding chapter, I will survey three relatively recent developments that undermine the single tax principle: tax competition for passive income, tax competition for active income, and tax arbitrage. I will then discuss various reactions to these developments at both the national and supranational levels (primarily through the OECD) and assess their success in curbing the threat to the single tax principle. Finally, I will discuss the implications of these reactions for the debate surrounding the existence of the international tax regime.