All economic sectors, including aviation, are experiencing growing pressure to reduce their greenhouse-gas (GHG) emissions. Aviation, however, has fewer alternative-energy options to petroleum-based fuels than other transportation sectors. Of alternatives that may be available (1) Fischer-Tropsch jet fuel produced from biomass or from a combination of coal and biomass with carbon capture and sequestration and (2) hydroprocessed renewable jet fuel may reduce aviation's impact on climate but are likely to be available only in limited quantities. Producing fuels yielding a net reduction in GHG emissions requires that biomass and renewable oil resources be produced so as not to incur land-use changes that would result in releases of carbon dioxide (CO2) and other GHGs. The economic benefits of producing alternative liquid fuels extend to all petroleum users. In particular, producing and using alternative liquid fuels yields benefits to commercial aviation, whether or not those fuels are used in aviation. Finally, moving to an ultra-low-sulfur specification for Jet A would reduce aviation's impact on air quality.