By means of Directive 2008/101 the EU extended its Emission Trading System to airline companies whose aircraft arrive at or depart from the territory of the EU Member States. Requested to provide a preliminary opinion on the validity of the Directive - especially in light of its extraterritorial application - the CJEU confirmed its effectiveness, arguably based mainly on the principle of sovereignty and only subsidiarily on the principle of environmental protection. In light of the interpretation provided by the CJEU, this paper critically assesses Directive 2008/101 and concludes that its consistency with international law should be considered in the light of the secondary consequences of the duty to protect the environment rather than territorial sovereignty.