On October 23, 2015, the EPA finalized Standards of Performance for Greenhouse Gas Emissions From New, Modified, and Reconstructed Stationary Sources: Electric Generating Units, found at 40 CFR part 60, subpart TTTT. That rulemaking determined that the best system of emission reduction (BSER) for greenhouse gases (GHGs) for newly constructed coal-fired steam generating units (i.e., EGUs) is efficient generation in combination with partial carbon capture and storage, the BSER for natural gas-fired base load combustion turbine EGUs is efficient generation (i.e., the use of combined cycle technology), and the BSER for non-base load and multi-fuel-fired combustion turbine EGUs is the use of clean fuels. On December 20, 2018, the EPA proposed to revised the standards of performance in 40 CFR part 60, subpart TTTT, but that proposal was never finalized. The EPA is undertaking a comprehensive review of the 111(b) GHG NSPS for EGUs, including of all aspects of the 2018 proposed amendments, the emission standard for base load combustion turbines, and the subcategorization for non-base load and multi-fuel-fired combustion turbine EGUs.