This introductory chapter explains the reasons behind the EU’s engagement with other international institutions. From a legal perspective, these reasons often relate to the division of competences between the Union and its Member States. The chapter addresses these competences as well as the challenges the EU faces on the basis of both its own rules and the rules of other international organizations. These rules result in a variety of ways in which the EU’s position in other institutions takes shape, from full membership to observer. The question of EU influence is only partly related to its formal position and is often linked to the Union’s ambitions to ‘uphold and promote its values and interests’ in the wider world.