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. On December 20, 2018, the EPA proposed to revise the standards of performance in 40 CFR Part 60, subpart TTTT. The EPA proposed to amend the previous determination that the best system of emission reduction (BSER) for newly constructed coal-fired steam generating units (i.e., EGUs) is partial carbon capture and storage, and replace it with a determination that BSER for this source category is the most efficient demonstrated steam cycle (e.g., supercritical steam conditions for large units and subcritical steam conditions for small units) in combination with the best operating practices. The EPA is undertaking a comprehensive review of the NSPS for greenhouse gas emissions from EGUs, including a review of all aspects of the 2018 proposed amendments and requirements in the 2015 Rule that the Agency did not propose to amend in the 2018 proposal.