The U.S. Environmental Protection Agency (EPA) is finalizing revisions to the Emission Guidelines for existing Municipal Solid Waste Landfills (MSW landfills). The EPA is not statutorily obligated to conduct a review of the Emission Guidelines (EG), but has the discretionary authority to do so when circumstances indicate that this is appropriate. Based on changes in the landfills industry and changes in size, ownership, and age of landfills since the Emission Guidelines were promulgated in 1996, the EPA has concluded that it is appropriate to review the landfills Emission Guidelines at this time. Since the 1980s, the number of active MSW landfills in the United States has decreased by approximately 75 percent and the share of sites that are publicly owned has also decreased substantially. The overall volume of disposal capacity has remained fairly constant, indicating a trend of growing individual landfill capacity at landfill sites. Based on our review of the existing MSW landfill industry, we are finalizing a lower annual non-methane organic compound (NMOC) emissions threshold of 34 megagrams1 per year (Mg/year) (reduced from 50 Mg/year). In addition, the EPA is finalizing New Source Performance Standards (NSPS) for new or modified Municipal Solid Waste Landfills. On July 17, 2014, the EPA proposed a new NSPS subpart that retained the same design capacity size threshold of 2.5 million meters cubed (m3) or 2.5 million Mg, but presented several options for revising the NMOC emission rate at which a MSW landfill must install controls. Since presenting these options, the EPA has updated its model that estimates the emission reduction and cost impacts based on public comments and new data. As a result of these data and model improvements, we `proposed to lower the annual NMOC emissions threshold from 50 Mg/year to 34 Mg/year in a supplemental proposal published August 27, 2015. This action finalizes the design capacity size threshold of 2.5 million m3or 2.5 million Mg and the annual NMOC emission threshold of 34 Mg/year.