Discusses fundamentals of income tax that are relevant to international commercial transactions, including identification of the fundamental features of payments as the building blocks of the income tax base. Considers the major sources of rules for taxation of international commercial transactions, including the OECD Model, the UN Model, EU direct tax law and their interaction with domestic tax rules. This consideration includes the coverage of tax treaties, their status, whether a dualist or monist approach to implementation is adopted as well as treaty override and treaty underride. There is a particular focus on how countries might implement rules so as to avoid the restrictions of tax treaties. The chapter moves to consider the interpretation of tax treaties including the relevance of the Vienna Convention on the interpretation of treaties and the status and relevance of the Commentary on the OECD Model. The abuse of tax treaties is also considered including its link with interpretation of tax treaties, the specific tax treaty GAAR based on principal purpose test and the interaction between tax treaties and domestic anti-abuse rules. In all of these matters there is a comparison with EU direct tax law, its implementation and interpretation.