The IAU is a member of the International Council for Science (ICSU). Its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership. The IAU has its head office on the second floor of the Institut d'Astrophysique de Paris in the 14th arrondissement of Paris. Working groups include the Working Group for Planetary System Nomenclature (WGPSN), which maintains the astronomical naming conventions and planetary nomenclature for planetary bodies, and the Working Group on Star Names (WGSN), which catalogs and standardizes proper names for stars. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center also operates under the IAU, and is a "clearinghouse" for all non-planetary or non-moon bodies in the Solar System. The Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers.
The IAU was founded on July 28, 1919, at the Constitutive Assembly of the International Research Council (now International Council for Science) held in Brussels, Belgium. Two subsidiaries of the IAU were also created at this assembly: the International Time Commission seated at the International Time Bureau in Paris, France, and the International Central Bureau of Astronomical Telegrams initially seated in Copenhagen, Denmark. The 7 initial member states were Belgium, Canada, France, Great Britain, Greece, Japan, and the United States, soon to be followed by Italy and Mexico. The first executive committee consisted of Benjamin Baillaud (President, France), Alfred Fowler (General Secretary, UK), and four vice presidents: William Campbell (USA), Frank Dyson (UK), Georges Lecointe (Belgium), and Annibale Riccò (Italy). Thirty-two Commissions (referred to initially as Standing Committees) were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets. The reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, May 2–10, 1922. By the end of the first General Assembly, ten additional nations (Australia, Brazil, Czecho-Slovakia, Denmark, the Netherlands, Norway, Poland, Romania, South Africa, and Spain) had joined the Union, bringing the total membership to 19 countries. Although the Union was officially formed eight months after the end of World War I, international collaboration in astronomy had been strong in the pre-war era (e.g., the Astronomische Gesellschaft Katalog projects since 1868, the Astrographic Catalogue since 1887, and the International Union for Solar research since 1904).
The first 50 years of the Union's history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents and General Secretaries. Twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Union's history in IAU Information Bulletin No. 100. Six past IAU Presidents in the period 1976–2003 also contributed their recollections in IAU Information Bulletin No. 104.
The IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide. 83% of all individual members are male, while 17% are female, among them the union's current president, Mexican astronomer Silvia Torres-Peimbert.
Membership also includes 79 national members, professional astronomical communities representing their country's affiliation with the IAU. National members include the Australian Academy of Science, the Chinese Astronomical Society, the French Academy of Sciences, the Indian National Science Academy, the National Academies (United States), the National Research Foundation of South Africa, the National Scientific and Technical Research Council (Argentina), KACST (Saudi Arabia), the Council of German Observatories, the Royal Astronomical Society (United Kingdom), the Royal Astronomical Society of New Zealand, the Royal Swedish Academy of Sciences, the Russian Academy of Sciences, and the Science Council of Japan, among many others.