Low carbon fuel standards (LCFSs) are rapidly becoming an integral part of the national debate over how to reduce greenhouse gas emissions. On January 18, 2007, California Governor Arnold Schwarzenegger signed an executive order launching a low carbon fuel standard to reduce the carbon intensity of fuels for light-duty vehicles.1 The standard, which limits an industry's weighted average carbon emissions rate or, equivalently, the carbon emissions per unit of output, allows fuel producers to achieve a given carbon emissions rate by flexibly altering their production of fuels.2 Senators John McCain and Barack Obama have both called for 1 The executive order S-01-07 states the LCFS shall "reduce the carbon intensity" of fuels. An accompanying white paper states that providers must "ensure that the mix of fuel … meets, on average, a declining standard for GHG emissions."2 There is much ambiguity in the design of an LCFS, including what output is covered (e.g., light-duty versus heavy-duty vehicle fuel), how output is measured (e.g., energy versus miles), and how emissions rates are measured (e.g., upstream versus downstream). These policy design issues are currently being debated for the California LCFS. ($60-$868). (JEL Q54, Q58)