The paper deals with the development of international organizations as recognized subjects of international law today. The precursors of international organizations first appeared in the 20th century BC. The paper traces further development during the Middle Ages and modern times, as well as the flourishing of international organizations after the two world wars. The League of Nations, as the first international universal organization, is described in more detail, followed by the most important international organization today, the United Nations. The paper also discusses the international legal personality of international organizations, theories explaining the emergence of international legal personality, and the consequences of this property in the domestic legal order of states, as well as in the international legal order. A document important for the recognition of the international legal personality of international organizations, the Advisory Opinion of the International Court of Justice from 1949, is also presented.