The book illustrates the fundamental aspects of international organizations by studying and analyzing their historical origins and classifications. Great attention is paid to the emergence and differences of international organizations. The book is mainly devoted to analyzing and discussing the concept of the legal personality of international organizations as well as detailing the operation and consequences resulting from the international legal personality. Comparing two theoretical approaches determining the legal personality of international organizations, differences and similarities are discussed in the examples of different international organizations by analyizing their constituent treaties. The unique feature of the paper is a comparative analysis of the universal and regional organizations in the matter of their international legal personality and the ways of acquisition in the examples of the United Nations as well as its Specialized Agencies, the European Union, and the Shanghai Cooperation Organization.