The Generation-IV consortium seeks to develop a new generation of nuclear energy systems for commercial deployment by 2020-2030. These systems include both the reactors and their fuel-cycle facilities. The aim is to provide significant improvements in economics, safety, sustainability, and proliferation resistance. The systems selected for development are the very high-temperature gascooled reactor (VHTR), the sodium-cooled fast reactor (SFR), the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), the molten salt reactor (MSR) and the super-critical water-cooled reactor (SCWR). UK organisations plan to contribute to the first three of these systems because of its existing capabilities and experience with gas-cooled systems, graphite cores, and SFRs. The science base for the VHTR and SFR systems is reasonably established, although there are gaps. For the VHTR, these include the performance of graphite at high neutron doses, and the performance of the fuel. For the SFR, the behaviour of fuels containing minor actinides, and processes for their recycling and refabrication into new fuel, must be established. The GFR presents many technical challenges, because it would need fuel and structural materials capable of withstanding extremes of fast neutron flux and high temperatures. Adequate heat removal from the core under fault conditions is likely to determine its feasibility. (C) 2008 Queen's Printer and Controller of HMSO. Published by Elsevier Ltd. All rights reserved.