ITER ("The Way" in Latin) is one of the most ambitious energy projects in the world today. In southern France, 35 nations* are collaborating to build the world's largest tokamak, a magnetic fusion device that has been designed to prove the feasibility of fusion as a large-scale and carbon-free source of energy based on the same principle that powers our Sun and stars. The experimental campaign that will be carried out at ITER is crucial to advancing fusion science and preparing the way for the fusion power plants of tomorrow. The primary objective of ITER is the investigation and demonstration of burning plasmas—plasmas in which the energy of the helium nuclei produced by the fusion reactions is enough to maintain the temperature of the plasma, thereby reducing or eliminating the need for external heating. ITER will also test the availability and integration of technologies essential for a fusion reactor (such as superconducting magnets, remote maintenance, and systems to exhaust power from the plasma) and the validity of tritium breeding module concepts that would lead in a future reactor to tritium self-sufficiency. Thousands of engineers and scientists have contributed to the design of ITER since the idea for an international joint experiment in fusion was first launched in 1985. The ITER Members—China, the European Union, India, Japan, Korea, Russia and the United States—are now engaged in a 35-year collaboration to build and operate the ITER experimental device, and together bring fusion to the point where a demonstration fusion reactor can be designed. We invite you to explore the ITER website for more information on the science of ITER, the ITER international collaboration and the large-scale building project that is underway in Saint Paul-lez-Durance, southern France. *Update 31 January 2021: In parallel with the Trade and Cooperation Agreement signed by the United Kingdom and the European Union on 24 December 2020, a Nuclear Cooperation Agreement (NCA) concluded between the UK and Euratom (the legal entity through which Europe holds its membership in ITER) makes clear the intent for the UK to remain a part of Fusion for Energy, the European Domestic Agency for ITER. In practical terms it means that the ITER Organization can expect the continued full involvement and participation of UK citizens and UK companies in ITER. The 35 nations participating in ITER are: the 27 European Union countries + (through Euratom) Switzerland and the United Kingdom + China, India, Japan, Korea, the Russian Federation, and the United States.