In July 2021, the European Commission published a proposal for a Carbon Border Adjustment Mechanism (CBAM), part of a wider package of laws aimed at implementing the EU Green Deal. The exact design of the CBAM is in flux, and priorities will have to be set. The chief concern is the compatibility of a CBAM with the law of the World Trade Organization (WTO). This Article explores whether and how the various CBAM design options under consideration can be reconciled with WTO requirements, focusing on a possible import border adjustment scheme. This issue’s Part One describes different instruments under consideration for the EU’s proposal; next issue, Part Two will assess the validity of these measures against the public policy exceptions contained in GATT Article XX. The measure will require careful design, and even then there is legal uncertainty in the WTO jurisprudence. In any event, the EU will be required to intensify its efforts to reach out to other jurisdictions to come to globally coordinated solutions.