This chapter examines the theoretical and conceptual foundations of this book. It begins by outlining and briefly discussing various scholarly perspectives on the EU’s diverse roles as an actor in international relations. Following this, it provides background on the EU actorness thesis and its application across different policy areas, regions, and countries. The chapter then debates the EU actorness thesis and its key elements, including presence, opportunity, and capability, drawing on the work of Bretherton and Vogler (The European Union as a global actor (1st ed.), Routledge, 1999, The European Union as a global actor (2nd ed.), Routledge, 2006, and International Relations, 27(3), 375–390, 2013). Finally, the last part highlights how the developed analytical framework, based on the EU actorness thesis, is applied in the subsequent chapters of the book.