Nuclear energy is attracting new interest around the world as countries look for low-carbon alternatives to fossil fuels to increase the diversity of their sources of energy and improve security of supply. Nuclear fission reactors provided approximately one sixth of the world's electricity needs in recent years. The vast majority of these reactors were built in the seventies and eighties. They are thus considered second generation systems, as they are based on experience gained with the first generation or prototypes built in the fifties and early sixties. Third generation reactors, developed in the nineties, are already a reality and will dominate the market in the coming decades. A significant research effort is underway on systems of the fourth generation. Better economics, improved use of natural resources, less production of radioactive waste, competitive production of hydrogen, and increased resistance to proliferation are within reach with these new systems. A review will be done on the most important features of third and fourth generation systems, together with a brief overview of the R&D challenges to be met. (C) 2010 Elsevier Ltd. All rights reserved.