This chapter presents the rules under EU law and international law that enable the EU to participate in the work of other international organizations. It explains the process of the EU becoming and acting as a full member in an international body composed of States, such as the Food and Agriculture Organization of the United Nations (FAO) or the World Trade Organization (WTO). When full membership is legally not possible, the EU is often granted the status of an observer. In this regard, the chapter presents pertinent examples from the UN and its specialized organizations. It puts particular emphasis on recent European Court of Justice (ECJ) case law, including Germany v Council (‘ OTIF I ’), where the Court underlined that the presentation of common EU positions is necessary in international organizations, even if the EU is not a full member thereof and has not legislated in the entire field of action covered by that organization.