ITER's roots lie in the need of the worldwide fusion development programme to build an experiment to succeed today's large experiments. This resonated with superpower politics in the mid-1980s, which needed to demonstrate concrete evidence of east-west collabration.

ITER's design has passed through a number of phases. A conceptual design phase led to the outline of an initial design. This was succeeded by a detailed engineering design phase initially on a larger device than the conceptual design, with a view to ensuring plasma ignition. This design was completed after 6 years joint work. Despite its large size, ignition proved difficult to ensure, and a less ambitious goal was established with a view to significant cost reduction. A device satisfying this need was completed in July 2001, to the extent necessary to establish its cost, schedule, safety, and licensing requirements. With the first site proposal from Canada, coordinated technical activities then continued to develop the design and to underpin negotiations on construction of ITER.