The United Nations was established on October 24, 1945 by 51 countries committed to preserving peace through international cooperation and collective security. Today, nearly every nation in the world belongs to the UN: membership totals 191 countries. The Headquarters consist of four main buildings: the General Assembly building, the Conference Building, the 39-floor Secretariat building, and the Dag Hammarskjold Library, which was added in 1961. The complex was designed by an international team of 11 architects, led by Wallace K. Harrison from the United States.