The International Thermonuclear Experimental Reactor (ITER) must provide an extensive test of concepts and technology required for the design and construction of a fusion reactor. This paper examines and offers some solutions to the critical issues related to a reactor, which include: development of a physics understanding of the approach to and control of ignition; choice of continuous or semi-continuous (long pulse) operation; development of a fully coherent and robust divertor for power exhaust and impurity control; minimisation of the effect of plasma disruptions; development of advanced materials and components needed for a reactor first wall and blanket, mechanical structure and superconducting coils; and development of technology for fast and reliable remote maintenance. The final challenge addressed is the integration of all these different reactor issues into a coherent and cost effective design. Worldwide collaboration, such as the ITER cooperation, is seen as the way to work most efficiently and to achieve effective solutions to these scientific and technological challenges.