This book systemically examines how the EU obtains and exercises its external competence and how this competence-exercising is overseen by legislature and reviewed by judiciary. The key question addressed in this book is, how the EU, as a supranational organisation and as new actor, participates in sovereign-states-based in international relations and reshapes international order. The exercise, oversight and review of EU external competence relates both to vertical distribution of competence between the EU and its Member States and to horizontal distribution of competence between different EU institution, including the European Commission, Council of the EU, European Parliament and Court of Justice of the EU. Secondly, as a regional economic integration organisation, the EU possesses an identity other than sovereignty states. Whether and how this unique identity contributes to a different international order which the EU is reshaping and reformulating is a central theme of this book. By examining four specific international regimes, the UNFCC, Rome Statute and International Criminal Court, Energy Charter Treaty and European Convention for the Protection of Human Rights and Fundamental Freedoms, this book appreciates the relevance of theories on the EU’s identity, including ‘EU as a normative power’, ‘EU as global actor’ and ‘EU as a rule-maker’. Through these cases, this book demonstrates a parallel between the acquisition of competence within the EU and externally its exercise in international relations.