THE TERM "international organization (IO)" generally refers to international governmental organizations or organizations with a universal membership of sovereign states. The most prominent IO is the United Nations, with 191 members. Other organizations whose membership is global include the World Trade Organization, the Universal Postal Union, and the International Hydrographic Organization. International organizations are established by treaties that provide legal status. International organizations are subjects of international law and are capable of entering into agreements among themselves and with member states.

Universal membership distinguishes international organizations from similar institutions that are open only to member states from a particular region. Examples of regional organizations include the European Union, the African Union, and the Organization of American States. These organizations are established by treaties among their members, enjoy international legal status, and can enter into agreements. There are still other organizations composed of member states that are based on particular criteria, such as historic association (the Commonwealth of Nations), economic development (the Organization for Security and Co-operation in Europe), and religion (Organization of the Islamic Conference).

Among international organizations, the United Nations has the longest and most significant experience with peace operations. International peacekeeping forces wearing blue helmets were first seen in the late 1940s. With a continuing surge in the demand for new peace operations, UN peacekeepers are now deployed in record numbers. In September 2006, the UNSC authorized a 40 percent increase in peacekeeping forces with the addition of 1,600 UN Police for East Timor and 13,000 new troops for southern Lebanon. The UNSC also authorized a 22,000-member peacekeeping force for Darfur, pending Sudanese government approval for the United Nations to replace the existing peacekeeping force of the African Union. The United States contributes police, but not troops, to UN peace operations.

This section of the guide describes the United Nations and its affiliated agencies, such as the World Health Organization, that compose the UN system. It describes a number of regional organizations that have played an important role in recent peace and stability operations. The section also describes a unique organization, the International Committee of the Red Cross (ICRC). ICRC is headquartered in Switzerland and staffed largely by Swiss nationals, but it has international legal status as a result of its responsibilities under the Geneva Conventions.