Lawsuits pending before the U.S. Court of Appeals for the D.C. Circuit are likely to shape the U.S. Environmental Protection Agency's (EPA's) authority to regulate greenhouse gas (GHG) emissions from stationary sources under the Clean Air Act (CAA). These legal challenges involve two high-profile CAA deregulatory actions: The EPA's Affordable Clean Energy rule ("Repeal of the Clean Power Plan; Emission Guidelines for Greenhouse Gas Emissions from Existing Electric Utility Generating Units; Revisions to Emission Guidelines Implementing Regulations," 84 Fed. Reg. 32520 (July 8, 2019)) (ACE Rule) and the EPA's rule eliminating the transportation and storage segments from and rescinding methane requirements in the new source performance standards for the oil and gas industry ("Oil and Natural Gas Sector: Emission Standards for New, Reconstructed, and Modified Sources Review Rule," 85 Fed. Reg. 57,018 (Sept. 14, 2020)) (policy Amendments rule) (collectively referred to as the rules). The rules repeal and replace Obama-era GHG emission regulations promulgated pursuant to Section 111 of the CAA, 42 U.S.C. Section 7411.