Considerable advances have been made in nuclear fusion research over many years and the stage was reached in 1988 for international collaboration between the European Communities, Japan, the USSR and the USA, under the auspices of the International Atomic Energy Agency. The aims of the collaboration are to develop the concept of an experimental fusion reactor to demonstrate the scientific and technological feasibility of fusion power. The Conceptual Design Activity (CDA) of the International Thermonuclear Experimental Reactor (ITER) started in early 1988 and was completed by the end of 1990. During the ITER-CDA the major physics and technological issues related to an ITER design were recognised and the work carried out on these issues provides the basis for the definition of the Next Step.