How are foreign relations constitutionally structured in federal unions? How does the foreign affairs constitution of the EU—itself a federal union in all but name—compare to that of other federal unions? This book addresses these questions. It offers a comparative analysis of the constitutional framework in which foreign relations are conducted in four federal unions: the US, Canada, Belgium, and the EU. The EU takes up a special position in the book. By looking at the constitutional experience of other federal unions—two long-standing (the US and Canada), one of more recent pedigree (Belgium)—this book informs and contributes to debates on how the EU’s foreign affairs constitution should be structured. Over a decade since the Treaty of Lisbon entered into force, the EU’s foreign affairs constitution continues to evolve. New institutional practices emerge and cases continue to be brought before the Court of Justice of the European Union that require the Court in Luxembourg to interpret Treaty provisions on how the EU and the Member States are to conduct foreign relations. In this sense, the EU’s foreign affairs constitution—as is the case for the EU’s constitution as a whole—remains very much a fast-moving target. The pace of constitutional change is high, and there is a sustained need for critical and constructive legal analysis of that change. This book contributes to fulfilling this need.