About two-thirds of primary energy today is used directly as transportation and heating fuels. Any discussion of energy-related issues, such as air pollution, global climate change, and energy supply security, raises the issue of future use of alternative fuels. Hydrogen offers large potential benefits in terms of reduced emissions of pollutants and greenhouse gases and diversified primary energy supply. Like electricity, hydrogen is a premium-quality energy carrier, which can be used with high efficiency and zero emissions. Hydrogen can be made from a variety of feedstocks, including natural gas, coal, biomass, wastes, solar sources, wind, or nuclear sources. Hydrogen vehicles, heating, and power systems have been technically demonstrated. Key hydrogen end-use technologies such as fuel cells are making rapid progress toward commercialization. If hydrogen were made from renewable or decarbonized fossil sources, it would be possible to have a large-scale energy system with essentially no emissions of pollutants or greenhouse gases. Despite these potential benefits, the development of a large-scale hydrogen energy infrastructure is often seen as an insurmountable technical and economic barrier. Here we review the current status of technologies for hydrogen production, storage, transmission, and distribution; describe likely areas for technological progress; and discuss the implications for developing hydrogen as an energy carrier.