The rational design of international institutions has been a prominent research agenda in the field of international relations. The chapter builds on this tradition by exploring how intergovernmental organizations (IGOs) design relationships with other IGOs. Using insights from organization theory on inter-organizational relations (IORs), it proposes to distinguish between the formalization, intensity, symmetry, and standardization of interactions between IGOs. The four dimensions allow for an analytical framework that is easy to operationalize. While the rational design literature shows great promise in analyzing the interaction between IGOs, it is important to account for the political nature of life in the international arena. The design of IORs in international relations is likely to be informal, intensive outside the formal settings, symmetrical and not very standardized.