The chapter outlines the legal concept of international organization. We map the debates on the traditional defining elements and the broader theoretical paradigms where the concept has been traditionally embedded. We argue that relatively well-established theoretical frameworks no longer match reality. Therefore we need a legal concept of international organization that is both sufficiently specific to have an analytical value for legal examination and sufficiently broad for not missing out entities which are apt to shape the normative situation of individuals or to deploy substantial direct or indirect legal effects for the fate of nations and for the integrity of our planet. We conceive of “international organization” as a cluster concept which does not depend on a set of fixed criteria. Some entities are in the core of the concept, others are more on the fringes. Besides actors which are inter-state in form, other entities with multiple legal bases (public and private, international and domestic) and with a hybrid membership (states, civil society, even commercial) may be qualified as international organizations if they are entrusted with competences to fulfil tasks in the global public interest and feature a certain degree of autonomy. The concept should also encompass actors devoid of legal personality when they are sufficiently structured and stable to distinguish them from mere networks and ad hoc cooperation. Ultimately and more radically, international law as part and parcel of a global legal landscape necessitates the concept of a global organization.