Growing international pressure to curb greenhouse gas (GHG) emissions has focused attention on existing policies that may, either by design or by effect, subsidize fossil fuel production and consumption. This paper reviews existing studies of fossil fuel subsidies within the United States, as well as assessments of the potential impact of subsidy reform on GHG emissions. Evaluating the differences across the studies, it highlights the most important disparities in subsidy definition and valuation in order to clarify the conclusions that can be drawn from this body of work. We then present some of the tools used to provide transparency in environmental regulation. We conclude that many of these approaches can be used to improve the transparency of fiscal policy, with important benefits within the context of climate change and beyond.