This report concerns the issue of methane leakage during the life cycle of natural gas-fired power generation. Methane leakage is an important issue because methane is a greenhouse gas (GHG) of much greater global warming potential than carbon dioxide (as much as 86 times greater over a 20-year time frame). Although natural gas-fired generation results in GHG emissions (almost exclusively carbon dioxide) of approximately one-half those associated with coal-fired generation, methane leakage at life cycle stages upstream of power generation (i.e., extraction, transmission) could result in GHG emissions over the fuel life cycle such that coal- and gas-fired generation are more similar in their total GHG emission profiles. Estimation of methane leakage is done using two principal approaches – the so-called bottom-up and top-down approaches. The top-down approach nearly universally yields methane leakage estimates greater than those derived from the bottom-up approach. This is a rapidly-evolving field, with new estimates published frequently. In addition to assessing our current understanding of the extent of leakage, SPSC contractor and report author M.J. Bradley & Associates covers key sources of methane emissions, along with potential control technologies and strategies, and federal and state policies to control emissions. Finally, in the report and in a separate document, implications for policymakers are discussed. This report provides an objective and thorough examination of available data for methane leakage over the life cycle of natural gas-fired power generation. It also presents technological and policy measures to reduce GHG-intensity of natural gas-fired power generation.