This report focuses on the mandated minimum usage requirements - referred to as the Renewable Fuel Standard (RFS) - whereby a minimum volume of biofuels is to be used in the national transportation fuel supply each year. It describes the general nature of the biofuels RFS and its implementation, and outlines some of the emerging issues related to the sustainability of the continued growth in U.S. biofuels production needed to fulfill the expanding RFS mandate, as well as the emergence of potential unintended consequences of this rapid expansion. This report does not address the broader public policy issue of how best to support U.S. energy policy.