Using biogas as a fossil fuel substitute can mitigate the buildup of greenhouse gases in the atmosphere. Therefore, biogas is an attractive renewable energy source in a carbon-constrained future. Although pipeline-quality biogas can augment the natural gas market supply, its long-term potential has been little studied. This report aims to answer the question of whether, and under what conditions, a substantial, decentralized domestic biogas market could develop in the United States by 2040. It finds that (1) biogas could be expanded to supply perhaps 3–5% of the total natural gas market at projected prices of $5–6/MMBtu, (2) the largest potential biogas is thermal gasification of agriculture and forest residues and biomass, (3) using biogas for electricity generation may be more profitable than supplying it to the pipeline, and (4) because market signals have not spurred widespread adoption of biogas, policy incentives are necessary to increase its use.