This chapter seeks to determine whether the Emissions Trading System of the European Union (EU-ETS) might be challenged or justified under the law of the World Trade Organization (WTO). In principle, much of the law of international air transport governed by the Chicago Convention is not covered by WTO law, but, arguably, certain important aspects may be covered. This chapter considers whether the principles of sustainable development might be used as justification of the EU-ETS should it be challenged under the WTO Dispute Settlement Understanding. It is argued that the principles of sustainable development would be invoked in WTO dispute settlement involving the EU-ETS.