organization of representatives of virtually all the states of Europe, as well as the United States and Canada, committed to formalizing decisions on important questions affecting the security and stability of the European continent as a whole. Its headquarters are in Vienna.

The organization was established in 1972, and its first conference (1973–75) was attended by all 33 countries of Europe (with the exception of Albania) and by the United States and Canada. The conference culminated in the signing on August 1, 1975, of the Helsinki Accords, in which the American- and Soviet-led alliances (the North Atlantic Treaty Organization and the Warsaw Pact, respectively) recognized the inviolability of the post-World War II frontiers in Europe and committed themselves to respect human rights and fundamental freedoms. Follow-up conferences were held in Belgrade, Yugoslavia (now in Serbia), in 1977–78; Madrid, Spain, in 1980–83; and Ottawa, Ontario, Canada, in 1985. Heads of state or government meet every two to three years.

After communist governments collapsed across eastern Europe in 1989 and the reunification of Germany became inevitable in 1990, a second major summit meeting was held in November in Paris to formally end the long-standing confrontation between the Western and Soviet blocs in Europe. The number of members was reduced from 35 to 34 by the reunification of Germany that October. The Paris summit was marked by the adoption of a Charter of Paris for a New Europe, which expanded the organization's role and established permanent institutions. In 1991 Estonia, Latvia, and Lithuania became members, and Russia assumed the seat held by the former Soviet Union. In 1992 the other republics formerly of the Soviet Union also became members, as did Albania.