Human activities around the world are producing increasingly large quantities of greenhouse gases, particularly carbon dioxide (CO2) resulting from the consumption of fossil fuels and deforestation. A comprehensive response to that problem would include a collection of strategies: research to better understand the scientific processes at work and to develop technologies to address them; measures to help the economy and society adapt to the projected warming and other expected changes; and efforts to reduce emissions. This report discusses the economic costs of reducing greenhouse-gas emissions in the U.S., describing the main determinants of costs, how analysts estimate those costs, and the magnitude of estimated costs. Illustrations.