A number of transportation fuels can be produced from renewable resources. The major fractions of lignocellulosic biomass, cellulose and hemicellulose, can be broken down into sugars that can be fermented into ethanol. Biomass can also be gasified to a mixture of carbon monoxide and hydrogen for catalytic conversion into methanol. Algae could consume carbon dioxide from power plants and other sources to produce lipid oil that can be converted into a diesel fuel substitute, Through anaerobic digestion, a consortium of bacteria can break down lignocellulosic biomass to generate a medium-energy-content gas that can be cleaned up for pipeline-quality methane. Catalytic processing of pyrolytic oils from biomass produces a mixture of olefins that can be reacted with alcohols to form ethers, such as methyl tertiary butyl ether (MTBE), for use in reformulated gasoline to reduce emissions. Each of these technologies is briefly described. The costs have been reduced significantly for biofuels, and the potential exists for them to be competitive with conventional fuels. An analysis of energy flows is presented for ethanol production as an example of these technologies, and a carbon dioxide balance is provided for the fossil fuels used. This analysis includes consideration of fuel utilization performance and assignment of carbon dioxide to coproduct. Biofuels technologies are shown to require little, if any, fossil fuel inputs. As a result, most or all of the carbon is recycled through their use, reducing substantially the net release of carbon dioxide to the atmosphere.