Multilateralism is a term in international relations that refers to multiple countries working in concert on a given issue.

International organizations, such as the United Nations (UN) and the World Trade Organization are multilateral in nature. The main proponents of multilateralism have traditionally been the middle powers such as Canada, Australia, Switzerland, the Benelux countries and the Nordic countries. Larger states often act unilaterally, while the smaller ones may have little direct power at all in international affairs aside from participation in the United Nations (by consolidating their UN vote in a voting bloc with other nations, for example). Multilateralism may involve multiple nations acting together as in the UN or may involve regional or military alliances, pacts, or groupings such as NATO.

The converse of multilateralism is unilateralism in terms of political philosophy.

One modern instance of multilateralism occurred in the nineteenth century in Europe after the end of the Napoleonic Wars where the great powers met to redraw the map of Europe at the Congress of Vienna. The Concert of Europe, as it became known, was a group of great and lesser powers that would meet to resolve issues peacefully. Conferences such as the Conference of Berlin in 1884 helped reduce great power conflicts during this period, and the 19th century was one of Europe's most peaceful.^{[citation needed]}in

Esther Brimmer is an American foreign policy expert and current Assistant Secretary of State for International Organization Affairs.

She received her D.Phil. (Ph. D.) and Master’s degrees in International Relations from the University of Oxford, UK, and her B.A. in International Relations from Pomona College in Claremont, California, USA.

Brimmer has served in the United States government and in international affairs think tanks. From 1999-2001 she was a Member of the Office of Policy Planning at the U.S. Department of State working on the European Union, Western Europe, the United Nations, and multilateral security issues. From 1995-1999 she managed projects as a Senior Associate at the Carnegie Commission on Preventing Deadly Conflict. She also served as a Special Assistant to the Under Secretary of State for Political Affairs (1993-1995) and as a Legislative Analyst at the Democratic Study Group in the United States House of Representatives (1991-1993). From 1989-1991 she was a management consultant with McKinsey.