The United Nations is both a possibility and a constraint for the EU. Multilateralism is a key value in the EU’s external action and the UN offers a platform to promote key EU interests in global diplomacy. The changes in the external representation architecture of the EU that were brought about by the Treaty of Lisbon necessitated a review of the EU’s participation in the UN. However, such review can only take place to the extent that it does not conflict with the rules that apply to the international institutions in question. The UNGA, the UNSC and the HRC therefore present three important case studies from the UN system where the Union adjusted its participation in accordance with its own rules and those of each organ, managing to have a consequential role and an impact on the work of these organs.