When the Renewable Fuel Standard (RFS) began mandating the use of corn ethanol in US fuels in 2006, many of its chief justifications were environmental. Commentators and policymakers assumed that some use of plant-based fuels would be less environmentally damaging than exclusive reliance on fossil fuels. Contrary to the optimism of early biofuel advocates, this paper documents a number of harmful environmental consequences of the Renewable Fuel Standard. The author finds that the Renewable Fuel Standard leads farmers to convert vast tracts of land, including previously unfarmed land, to corn farming and to unsustainably intensify production on existing farms. The author also finds that thecorn needed to fulfill the ethanol mandate demands heavy fertilization and irrigation in areas prone to fertilizer-linked water quality issues and drought, but fails to produce much net energy. Finally, the research concludes that ethanol is not meaningfully superior to gasoline in terms of carbon dioxide emissions and is worse for other pollutants. This report illuminates these and other problems with the widespread use of corn ethanol prompted by the Renewable Fuel Standard and makes recommendations for how to mitigate these impacts.