Fossil fuel-fired electric generating units (EGUs) are the nation’s second largest source of greenhouse gas (GHG) pollution. Under CAA section 111(b), EPA sets New Source Performance Standards (NSPS) for new, modified, and reconstructed stationary sources. In 2015, EPA finalized regulations to limit GHG emissions from new fossil-fuel fired steam generating units (e.g., utility boilers) and from natural gas-fired stationary combustion turbines (40 CFR part 60 subpart TTTT). On August 3, 2015, EPA promulgated its first emission guideline regulating greenhouse gases (GHGs) from existing fossil fuel-fired electric generating units (EGUs) in the Clean Power Plan (CPP) (40 CFR part 60 UUUU), which was subsequently stayed by the U.S. Supreme Court. On June 19, 2019 EPA issued a new rule, the Affordable Clean Energy (ACE) Rule (40 CFR part 60, subpart UUUUa) and a repeal of the CPP. On January 19, 2021, the D.C. Circuit Court vacated the ACE Rule and remanded the rule to EPA for further consideration consistent with its decision. The U.S. Supreme Court then overturned the D.C. Circuit Court’s decision in the WV v EPA opinion in June 2022. EPA is now undertaking new rulemakings to amend the GHG NSPS under CAA section 111(b) and establish emission guidelines under CAA section 111(d) to limit GHG emissions from existing fossil fuel-fired EGUs. EPA anticipates issuing proposed rules for these actions in Spring 2023, and promulgating final rules by Spring 2024.