The European Union has gradually developed a comprehensive body of legislation aimed at protecting the environment, consolidating its competences in this field in successive stages. Within this framework, climate change has been given a high priority and is one of the most prominent areas of the EU’s external and internal policy. In a fruitful interplay, the EU has driven the evolution of the international 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 the EU climate policy facing conflicting challenges of ambition, acceptability and effectiveness.