This article reviews recent congressional legislation promulgating regulations to reduce transportation’s greenhouse gas (GHG) emissions. It points out that, while progress in the 111th Congress is uncertain, the Clean Air Act requires the Environmental Protection Agency (EPA) to regulate GHG emissions, and that both the EPA and the U.S. Department of Transportation are drawing on current laws to mandate reduced GHG emissions per mile traveled by passenger vehicles and to sharpen the definitions and criteria for renewable fuels and feedstocks.