As the saying goes, "Be careful what you wish for; you might get it." This describes the current state of biofuels. The appealing concept of biofuels is supported by well-meaning environmental advocates, business interests, and governmental policy, but if we don't make wise choices, then the reality may not be all that we wished for. This article examines factors that relate to the sustainability of the use of biofuels to partly replace some petroleum energy sources for transportation. Key inquiries concern first the energy and emissions balance of the use of biofuels when compared to petroleum and, second, the broader environmental impact of the biofuel production. One form of biofuel in particular, cellulosic ethanol, is a strong contender for the role as a sustainable fuel. Other forms under serious consideration are corn-based ethanol and biodiesel. This article focuses on ethanol as a sustainable transportation fuel.