O n October 23, 2015, the U.S. Environmental Protection Agency (EPA) published its final Clean Power Plan rule (CPP or Rule) to regulate emissions of greenhouse gases (GHGs), specifically carbon dioxide (CO 2 ), from existing fossil fuel-fired power plants. The goal of the Rule, according to EPA, is to help protect human health and the environment from the impacts of climate change. The CPP has been one of the more singularly controversial environmental regulations ever promulgated, and the controversy is reflected in the enormous multi-party litigation over the Rule in the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit). Numerous petitions challenging the Rule have been consolidated under the caption West Virginia v. EPA . While the litigation is still ongoing at the circuit court level, an interlocutory—that is, mid-litigation—application to the Supreme Court resulted in a stay, or pause, of the Rule. This report provides legal background on the Rule, its Clean Air Act (CAA) framework under Section 111, and some of the main climate-related lawsuits that have preceded the present litigation over the CPP. It then gives an overview of the participants in the litigation, including Members of Congress, who have offered briefs in support of both sides. This report explains the major events in the litigation as of the date of publication, including the Supreme Court stay and the D.C. Circuit oral argument, and the expected schedule of events in the near term. It then presents condensed summaries of some of the main arguments on the merits. This report concludes with a brief look at parallel litigation in the D.C. Circuit that is challenging a related EPA regulation, which limits GHG emissions from new, modified, and reconstructed power plants. The CAA encompasses a number of program authorities, all with the general aim of protecting human health and the environment from emissions that pollute ambient air. Debate over the use of the CAA to regulate GHG emissions has its origins at least as far back as 1999, when several groups filed a petition urging EPA to regulate GHG emissions from new motor vehicles and motor vehicle engines under CAA Section 202. EPA denied the petition in 2003 after soliciting public comments. Shortly thereafter, some of the groups were joined by 12 states and others in filing a petition for review of EPA's decision in the D.C. Circuit. The D.C. Circuit, in a fractured opinion, deferred to EPA's denial of the petition. On appeal, however, in its 2007 decision in Massachusetts v. EPA , a five-Justice majority of the Supreme Court held that EPA has statutory authority to regulate GHG emissions under CAA Section 202(a)(1), which requires the EPA Administrator to set emission standards for "any air pollutant" from motor vehicles "which in his judgment cause[s], or contribute[s] to" air pollution which "may reasonably be anticipated to endanger public health or welfare." GHGs, the Court said, unambiguously fell within the broad definition of "air pollutant." The Court also found that EPA had acted arbitrarily and capriciously in explaining its denial of the petition. Citing the Massachusetts v. EPA decision, EPA issued an "endangerment" finding and a "cause or contribute" finding in December 2009. These findings formed the basis for the light-duty vehicle GHG emission standards and corporate average fuel economy (CAFE) standards issued jointly by EPA and the National Highway Traffic Safety Administration (NHTSA) in 2010. In 2011, in American Electric Power Co. v. Connecticut (" AEP "), the Supreme Court unanimously held that EPA's authority to regulate GHG emissions under the CAA—including its power under Section 111(d), the basis of the CPP—displaced any common law tort or nuisance claims against power plants and other GHG emissions sources. The Court in AEP explicitly ruled that "air pollutant" includes GHGs when applied to power plants under Section 111, as under Section 202 for motor vehicles. The Court concluded that federal judges may not set limits on GHG emissions because the CAA "empower[s] EPA to set the same limits," and therefore did not allow the plaintiffs, including states, to proceed with their lawsuits against power plant operators. With GHGs being regulated under CAA Section 202, EPA proceeded with regulating GHGs under other CAA authorities for stationary sources. In particular, EPA interpreted the mobile source GHG regulations as triggering regulations under the Prevention of Significant Deterioration (PSD) program. The PSD program generally requires new or modified stationary sources that will emit threshold amounts (250 or 100 tons per year depending on the type of source) of air pollutants subject to regulation under the CAA to obtain permits and comply with emissions limitations that reflect the "best available control technology" (BACT). EPA likewise sought to regulate GHGs under the Title V permit program. Title V requires permits for "major sources" with the potential to emit 100 tons per year of any air pollutant. As EPA noted, GHG emissions tend to be "orders of magnitude greater" than emissions of other types of air pollutants, so the statutory thresholds would have swept in many smaller sources not previously subject to CAA permitting. EPA addressed this by issuing a "tailoring rule," structured to phase in GHG permitting under the PSD and Title V programs first for "anyway" sources already subject to permitting for other air pollutants, and then to non-anyway sources meeting higher thresholds. In 2014, in Utility Air Regulatory Group v. EPA (" UARG "), the Supreme Court rejected EPA's interpretation of the "triggering" provisions for the stationary source programs; it held that EPA cannot regulate a power plant solely due to its GHG emissions, striking down EPA's "tailoring" rule. Justice Scalia, writing for a five-Justice majority of the Court, stated the following: EPA's greenhouse-gas-inclusive interpretation of the PSD and Title V triggers … [is] unreasonable because it would bring about an enormous and transformative expansion in EPA's regulatory authority without clear congressional authorization. When an agency claims to discover in a long-extant statute an unheralded power to regulate "a significant portion of the American economy," we typically greet its announcement with a measure of skepticism. On the other hand, in a part of the decision joined by seven Justices, the Supreme Court affirmed EPA's authority under the CAA to require Best Available Control Technology (BACT) to regulate GHG emissions from power plants if the source is regulated for other air pollutants, holding EPA's interpretation of such requirements reasonable. In sum, UARG held that EPA "may not treat greenhouse gases as a pollutant for purposes of defining a 'major emitting facility' … in the PSD context or a 'major source' in the Title V context.... EPA may, however, continue to treat greenhouse gases as a 'pollutant subject to regulation under this chapter' for purposes of requiring BACT for 'anyway' sources." As a practical matter, UARG affirmed EPA's ability to regulate roughly 83% of U.S. stationary-source GHG emissions under PSD and Title V permitting programs, and struck down its ability to regulate the additional 3% that would have been reached had the tailoring rule been upheld. In 2011, EPA finalized a settlement agreement with states and others to promulgate New Source Performance Standards (NSPSs) for GHG emissions from fossil fuel-fired power plants under Section 111(b) of the CAA, and emission guidelines covering existing power plants under Section 111(d). President Obama also directed EPA to issue GHG regulations under Section 111(b) and 111(d) in a presidential memorandum issued in June 2013. As characterized by EPA, Section 111 operates to address one of three "general categories of pollutants emitted from existing stationary sources," the other two being (1) "criteria" air pollutants under the National Ambient Air Quality Standards (NAAQS) program under CAA Sections 108-110; and (2) "hazardous air pollutants" (HAP) under the National Emission Standards for Hazardous Air Pollutants (NESHAP) program under CAA Section 112. Section 111 addresses "air pollution which may reasonably be anticipated to endanger public health or welfare." Section 111 directs EPA to list categories of stationary sources that cause or contribute significantly to such air pollution; to establish NSPSs for new sources within any such category; and then to issue a "procedure" requiring states to submit plans that establish standards of performance for existing sources in a category, under certain conditions. In other words, NSPSs under Section 111(b) may trigger what EPA terms "emission guidelines" under Section 111(d). Portions of CAA Section 111 primarily relevant to the CPP litigation are excerpted below (with indentations and bracketed notations added for readability): (a) Definitions. For purposes of this section: (1) The term "standard of performance" means a standard for emissions of air pollutants which reflects the degree of emission limitation achievable through the application of the best system of emission reduction [BSER] which (taking into account the cost of achieving such reduction and any non-air quality health and environmental impact and energy requirements) the Administrator determines has been adequately demonstrated. … (3) The term "stationary source" means any building, structure, facility, or installation which emits or may emit any air pollutant.... … (b) (1) (A) The Administrator shall … publish (and from time to time thereafter shall revise) a list of categories of stationary sources. He shall include a category of sources in such list if in his judgment it causes, or contributes significantly to, air pollution which may reasonably be anticipated to endanger public health or welfare.