This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book suggests that European Union (EU) law is playing a key role in the democratisation of EU international relations. It tackles more precisely the dual source of legitimacy by focusing on national parliaments in the context of mixed agreements and whether they are indeed rebels with a cause. The book aims to frame the discussion on the basis of basic concepts such as institutional balance and democracy. It focuses on the sources of legitimacy of the EU's foreign affairs powers and the role of the Court of Justice in the patrol of institutional balance and the democratisation of EU international relations. The book conducts a comprehensive analysis of the significant role that the principle of institutional balance has played in the post-Lisbon jurisprudence on external action. It focuses specifically on the future of mixed agreements after the Treaty of Lisbon.