Federal policy has played a key role in the emergence of the U.S. biofuels industry. Policy measures have included minimum renewable fuel usage requirements, blending and production tax credits, an import tariff, loans and loan guarantees, and research grants. One of the more prominent forms of federal policy support is the Renewable Fuel Standard (RFS) -- whereby a minimum volume of biofuels is to be used in the national transportation fuel supply each year. This book describes the general nature of the RFS mandate and its implementation, and outlines some emerging issues related to the continued growth of U.S. biofuels production needed to fulfill the expanding RFS mandate, the potential inability of the domestic market to absorb ethanol above a 10% share of domestic gasoline fuels (a problem known as the "blend wall"), and the emergence of potential unintended consequences of this rapid expansion.