In the development of advanced gas-cooled, light-water-cooled, or heavy-water-cooled reactors, there are many challenges facing the materials engineer and the designer. High-temperature gas-cooled reactors offer considerable potential as efficient generators of electricity and producers of high quality process heat. Design innovations and further alloy development will be needed to overcome the many materials problems created by their high temperature of operation. One of the most significant problems in light water reactor development has been steam generator failures. By careful choice of materials, water chemistry control, and design, the problems can be minimized and perhaps eliminated. Heavy water reactors are particularly suited for the development of thorium fuel cycles, which can be used to extend our fuel resources. The strict neutron economy needed to successfully operate a self-sufficient or near-breeding thorium fuel cycle requires the continued development of improved core materials and imaginative design in their application.