It is the stated policy of the UK government to reduce emissions of carbon dioxide by 60% by 2050. This policy, which goes far beyond commitments under the Kyoto agreement, was originally advocated by the Royal Commission on Environmental Pollution, of which the author was a member. Its acceptance was seen by many as a surprising development, possibly reflecting the strength of the underlying case. The target was developed by a three-legged argument which reflects the three components of sustainability: Environmental constraints: limits on emissions to avoid risk of major climate change; Social equity: equal per-capita allocation of emissions; Techno-economic: the feasibility and cost of reduction on this scale. Assessment of techno-economic feasibility shows that the target can be achieved economically if the efficiency of energy use is improved to achieve reduction in demand, combined with a shift to lower-carbon energy sources. The greatest scope for demand reduction lies in improving the building stock, combined with providing low-grade heat from sources such as biomass. On the supply side, the principle questions are how much controllable electricity generation is needed, and whether this capacity should be nuclear or fired by fossil fuels with the carbon dioxide formed sequestered in geological strata. Increased use of biomass is a key part of the shift to a lower carbon economy: the barriers which have retarded the development of biomass in the UK are explored. (c) 2006 Elsevier Ltd. All rights reserved.