Based on a thorough investigation of the existing law, and a deep understanding of its historical and economic rationale, it offers a remarkably sound and well-balanced analytical framework to define the exact limits in international law to the application of cross-border taxes, namely for environmental purposes. The research manages to stay resolutely away from (unfounded) ideological reasoning, which would either blindly support interpretations rejecting the very idea that cross-border trade should be the object of specific taxes, or systematically support sovereignty arguments, which would undermine the legitimacy of supranational rules.