The Environmental Protection Agency (EPA) is finalizing a new subpart that updates the Standards of Performance for Municipal Solid Waste Landfills. Under section 111 of the Clean Air Act, the EPA must review, and, if appropriate, revise standards of performance at least every 8 years. The EPA's review of the standards for municipal solid waste landfills considered landfills that commence construction, reconstruction, or modification after July 17, 2014. The final standards also reflect changes to the population of landfills and an analysis of the timing and methods for reducing emissions. This action will achieve additional reductions in emissions of landfill gas and its components, including methane, by lowering the emissions threshold at which a landfill must install controls. This action also incorporates new data and information received in response to the proposed rulemaking and addresses other regulatory issues including surface emissions monitoring, wellhead monitoring, and the definition of landfill gas treatment system. The new subpart will reduce emissions of landfill gas, which contains both nonmethane organic compounds and methane. Landfills are a significant source of methane, which is a potent greenhouse gas pollutant. These avoided emissions will improve air quality and reduce the potential for public health and welfare effects associated with exposure to landfill gas emissions.