R. Mayson, A, Worrall, K. Hesketh - 2004: Written for the Institute of Physics, this article explains how present day nuclear reactors work, and what improvements are being considered. The 440 fission reactors around the world have a good and improving record for performance and safety. These facts are stimulating new orders for new plants. Anticpating these opportunities, designers are studying better ways to build these systems. Some of the proposals are evolutionary, offering simplifications that reduce costs. Others are revolutionary, offering significant efficiency and sustainability gains. The evolutionary developments are being worked out for thermal reactors similar to those in use today. Costs can be reduced by using simpler means for safety shutdown, by using less expensive components such as light water instead of heavy water as a coolant, and by employing standardized production procedures. At the same time, the Generation IV research initiative is investigating six advanced designs with fast neutron flux, closed fuel cycles, various fission fuels, and modular construction. Other topics such as actinide transmutation are also explained. The article ends with a recommendation that we take full advantage of these engineering initiatives to reduce the global warming threat.