Coal power plants generate about half of the United States' electricity and are expected to remain a key energy source. Coal power plants also account for about one-third of the nation's emissions of carbon dioxide (CO2), the primary greenhouse gas that experts believe contributes to climate change. Current regulatory efforts and proposed legislation that seek to reduce CO2 emissions could affect coal power plants. Two key technologies show potential for reducing CO2 emissions: (1) carbon capture and storage (CCS), which involves capturing and storing CO2 in geologic formations, and (2) plant efficiency improvements that allow plants to use less coal. The Department of Energy (DOE) plays a key role in accelerating the commercial availability of these technologies and devoted more than $600 million to them in fiscal year 2009. Congress asked GAO to examine (1) the maturity of these technologies; (2) their potential for commercial use, and any challenges to their use; and (3) possible implications of deploying these technologies. To conduct this work, GAO reviewed reports and interviewed stakeholders with expertise in coal technologies. GAO recommends that DOE develop a standard set of benchmarks to gauge and report to Congress on the maturity of key technologies.