Bioenergy, including biofuels, could become a substantial tool for mitigating greenhouse gas emissions, locally and globally, possibly providing a large fraction of global primary energy supply by 2020. Exactly how large that share will be is not possible to predict with any precision, being dependent on a complex array of physical, social, economic, technical (innovation) and environmental factors. In addition, there will be competition for biomass resources between the different bioenergy sectors (electricity, heat, transport) and alternative uses e.g. for chemical feedstocks and materials. There will be synergies too, particularly arising through advanced polygeneration and biorefinery supply chains that could help to raise primary productivity and raise resource-use-efficiencies. However, assessing the actual environmental impacts of increased bioenergy and in particular, biofuel usage, will depend sensitively on the scale and mix of technology options employed and on the location. Location is important the fundamental factors that govern biomass productivity vary significantly according to site e.g. soil type, climate, including water availability and temperature. Across a range of indicators, one biofuel may not be the same as another, even where the final fuels are chemically and physically identical e.g. anhydrous ethanol derived from wheat, sugarcane, sugar beet, cassava or from residues.