This chapter will tackle two basic questions: i) whether and, if so, to what extent international environmental principles and rules are relevant to the interpretation and application of EU environmental law; and ii) whether international law is relevant to issues of EU competence, both internally and externally, in the field of environmental protection, with specific reference to the problem of distribution of competencies between the EU and its member states and the practice of mixed environmental agreements. The chapter will first remind that international law, especially after the Lisbon Treaty, is most relevant both to the interpretation and application of EU environmental law and to the way EU law deals with issues of competence and ...