This article contends that an international tax regime exists, embodied in both the tax treaty network and the domestic tax laws of the major trading nations. The article also contends that the international tax regime is an important part of international law as it evolved in the 20th century - in particular, that parts of international tax law can be seen as customary international law and therefore as binding even in the absence of treaties. The article also explains the basic structure of the international tax regime and its underlying norms - the single tax principle and the benefits principle - and sets out the normative rationale for each. Finally, the article discusses recent challenges to the international tax regime, such as tax competition and tax arbitrage, and argues that the reaction to these challenges by the OECD, the WTO, the European Union and specific tax administration proves the existence of the international tax regime.