The European Union has gradually developed a comprehensive body of environmental legislation. Within this framework, climate change has been given a high priority and is a prominent area of the EU’s external and internal policy. In a fruitful interplay, the EU has driven the evolution of the international climate regime and tried to influence international negotiations. In turn, the international regime has urged the Union to establish common objectives and means to tackle climate change in order to implement its international commitments. EU legislation and the international climate regime have been in constant interaction and have evolved together. Their interactions played a major role in the shaping of EU regulatory climate action facing conflicting challenges of ambition, acceptability and effectiveness.