The substitution of biogas, an energy source derived from biological feedstock, for fossil natural gas (NG) can mitigate the build-up of greenhouse gases in the atmosphere, making it an attractive renewable energy source in a carbon-constrained future. Although upgraded, pipeline-quality biogas can augment the NG market supply in the United States, researchers and energy industry experts have little studied its long-term potential. This article estimates (1) levelized costs of energy for biogas production facilities operating with landfill waste, animal manure, wastewater sludge, and biomass residue feedstocks; (2) feedstock and technology pathway-specific biogas supply functions; and (3) the aggregate national biogas supply potential for the United States by 2040. Under a range of specified assumptions, generation of biogas could be expanded to approximately 3–5 percent of the total domestic NG market at projected prices of $5–6/MMBtu; the largest potential source comes from thermal gasification of agriculture and forest residues and biomass. As market signals have not spurred widespread adoption of biogas in the United States, policy incentives similar to those used in the European Union may be necessary to increase its production and use. Bioenergy policy in the European Union and the resulting market penetration achieved there provides important lessons for how biogas markets in the United States can overcome barriers to market expansion and, in doing so, provide substantial climate mitigation benefits.