Political groups permeate the diplomatic process across the United Nations (UN) system, from conference diplomacy to annual sessions of the deliberative bodies, yet they remain poorly understood and under-appreciated. This article approaches groups from a conceptual and theoretical perspective, providing a typology to differentiate clearly the various groups that are active in UN processes, from electoral groups to regional organizations and single-issue coalitions. The article also examines how theories of multilateralism, global governance and international negotiation largely exclude group and inter-group dynamics. Theories of global governance and multilateralism operate at the systemic level of analysis, while theories of negotiation and coalitions reflect assumptions of individual agency; both levels of analysis obscure the operation of political groups and group politics in UN multilateralism. The emerging theories of diplomatic practice provide a meso-level approach that reveals the pervasive practice of group politics and politicized diplomacy in UN multilateralism.