The International Space Station partnership includes 15 nations. They cooperate through five principal space agencies: NASA, Roscosmos, the European Space Agency, Japan’s JAXA, and the Canadian Space Agency.
The partnership was formalized by the International Space Station Intergovernmental Agreement, signed in 1998. The participating governments included the United States, Russia, Canada, Japan, and 11 European countries represented through ESA: Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland, and the United Kingdom.
A common mix-up is to count agencies rather than nations. The ISS has five major partner agencies, but those agencies represent 15 countries. ESA’s role is especially important because it is a multinational organization rather than a single country’s space program.
The partnership covers station construction, operations, research, crew flights, and use of laboratory facilities. Astronauts from many other countries have visited the ISS, but visitors and experiment participants should not automatically be counted as formal partner nations.