On 14 July 2021 the European Commission published a proposal for a Carbon Border Adjustment Mechanism (CBAM). The authors discuss the background and legal structure of what the Commission has presented as a climate measure. They also provide detailed commentary on the provisions in the current proposal, including discussion of the interaction between customs law and the proposed CBAM. Lastly they consider the international reactions to the CBAM, as well as CBAM initiatives elsewhere in the world.