Fossil fuel-fired power plants are the nation’s second largest source of GHG pollution. 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 (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 Rule (40 CFR part 60, subpart UUUUa) and a repeal of the Clean Power Plan. On January 19, 2021, the D.C. Circuit Court vacated the Affordable Clean Energy Rule and remanded the rule to EPA for further consideration consistent with its decision. On February 12, 2021, considering the D.C. Circuit’s decision, the EPA published a memorandum on the status of the Affordable Clean Energy rule and informed states not to continue the development or submittal of state plans in accordance with CAA section 111(d) guidelines for GHG emissions from power plants at this time. The U.S. Supreme Court then overturned the D.C. Circuit’s decision in the WV v EPA opinion in June 2022. EPA is considering the implications of this U.S. Supreme Court decision and is now undertaking a new rulemaking to establish emission guidelines under CAA 111(d) to limit GHG emissions from existing fossil fuel-fired EGUs. EPA anticipates issuing a proposed rule for this action in Spring 2023, and promulgating a final rule by Summer 2024.