On March 28, 2017, President Trump issued Executive Order 13783, which aims to promote the development or use of domestically produced energy resources. Among its specific provisions, the order rescinded the Obama Administration's "Climate Action Plan" (CAP). The CAP aimed to reduce emissions of carbon dioxide (CO 2 ) and other greenhouse gases (GHGs), as well as to encourage adaptation to a changing climate. One of the interagency initiatives within the CAP, the "Strategy to Reduce Methane Emissions" (Methane Strategy), focused on the control of methane emissions, a short-lived climate pollutant with a Global Warming Potential of more than 25 times CO 2 . President Trump's order also requires agencies to review their existing regulations and "appropriately suspend, revise, or rescind those that unduly burden" domestic energy production and use. The order specifically requires the U.S. Environmental Protection Agency (EPA) to review the revised volatile organic compounds (VOCs) and GHG (namely methane) emission standards for new, modified, and reconstructed equipment, processes, and activities of the oil and natural gas sector issued by the Obama Administration in June 2016. EPA issued these emission standards pursuant to Section 111 of the Clean Air Act (CAA) and the Methane Strategy. Upon review of the standards and several administrative petitions for reconsideration, EPA has now decided to reconsider certain requirements in the emissions standards for the oil and natural gas sector. Although the methane emission standards and guidelines for new and existing municipal solid waste (MSW) landfills are not specifically mentioned in Executive Order 13783, EPA has also reviewed and is currently reconsidering several requirements of the MSW landfill emission standards and guidelines that the Obama Administration updated in August 2016. EPA's review of the oil and natural gas sector and landfill methane rules has influenced several judicial challenges to the Obama-era rules. The U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit) granted EPA's requests to pause the judicial challenges to allow EPA to complete its review of the regulations. In addition, EPA has attempted to stay the requirements that are being reconsidered by the agency, but courts have restricted the agency's ability to stay such requirements. This report examines the statutory authority for issuing the various methane regulations, legal challenges to the standards, and legal issues related to the reconsideration and stay of these regulations. The report begins by providing a general legal background of CAA Section 111 and Section 307 requirements for reconsidering rules, before addressing the oil and natural gas sector rules and the MSW landfill methane rules. EPA issued the regulations addressing methane emissions from oil and natural gas sources and MSW landfills pursuant to CAA Section 111. Section 111 directs EPA to list categories of stationary sources that cause or contribute significantly to "air pollution which may reasonably be anticipated to endanger public health or welfare." Once EPA lists a source category (e.g., "crude oil and natural gas production"), Section 111(b) requires EPA to establish "standards of performance" (known as new source performance standards, or NSPSs) for new, modified, and reconstructed sources within a listed source category. A "standard of performance" is defined as an air pollution emission standard that reflects the "best system of emission reduction" (BSER) that has been "adequately demonstrated." In determining the BSER, EPA must take into account the cost of achieving the emission reductions; non-air quality health and environmental impacts; and energy requirements of the regulated sources. At the same time or after the Section 111(b) standards are issued for new , modified , and reconstructed sources, EPA must issue a "procedure" requiring states to submit plans that establish standards of performance for existing sources in a source category, under certain conditions. EPA refers to these Section 111(d) procedures as "emission guidelines" that states are required to follow when they develop plans to implement standards of performance for existing sources in their jurisdictions. Congress established a single definition for standards of performance promulgated pursuant to Sections 111(b) and 111(d), requiring EPA to consider the same factors when establishing standards of performance for both new, modified, reconstructed sources and existing sources. Section 111(d)(1)(A) limits EPA's authority to issue Section 111(d) emission guidelines for existing sources. Based on its interpretation of Section 111 under the Obama Administration, for any air pollutant to which a Section 111(b) NSPS applied for new, modified, or reconstructed sources, EPA was required to regulate the same pollutant under Section 111(d) for existing sources unless that air pollutant was already regulated as (1) a "criteria pollutant" under the National Ambient Air Quality Standards (NAAQS) program or (2) a "hazardous air pollutant" (HAP) under the National Emission Standards for Hazardous Air Pollutants (NESHAP) program. For example, because GHGs such as methane are not considered "criteria pollutants" under the NAAQS or HAPs under the CAA, EPA reasoned that it had authority under Section 111(d) to address methane emissions from existing MSW landfills once it had issued NSPSs for methane emission standards for new, modified, and reconstructed sources in those categories. However, EPA's interpretation and scope of its authority under Section 111(d) are currently subject to judicial challenge. Prior to 2016, EPA had issued Section 111(d) emission guidelines to address the following: GHG emissions from existing fossil fuel-fired power plants (known as the Clean Power Plan, or CPP); VOC and methane emissions from MSW landfills; organics, metals, and nitrogen oxides from municipal waste combustor units; acid mist from sulfuric acid production units; air pollutants from hospital/medical/infectious waste incinerators; fluoride emissions from phosphate fertilizer plants; reduced sulfur emissions from kraft pulp mills; and fluoride emissions from primary aluminum plants. Under CAA Section 307(d)(7)(B), EPA must convene a reconsideration proceeding if the petitioner meets certain conditions. The petitioner must demonstrate that the objection to the final rule could not have been raised during the public comment period for the proposed rule, and the EPA Administrator must conclude that the "objection is of central relevance to the outcome of the rule." In granting reconsideration, EPA must provide the "same procedural rights as would have been afforded had the information been available at the time the rule was proposed," such as public notice and comment. If EPA denies the petition for reconsideration, the petitioner may seek judicial review of the denial. Further, under Section 307(d)(7)(B), EPA's reconsideration of a rule does not automatically postpone the effectiveness of the rule unless EPA or a court stays the rule for up to three months during the reconsideration. In other words, if EPA determines that it must convene a reconsideration proceeding, the agency or a court may grant a stay of the effectiveness of the rule for a limited period of three months. As discussed below, EPA has granted reconsideration for specific requirements in the oil and natural gas sector and landfill methane rules and has issued three-month stays of the effective dates of those requirements for the rules. The crude oil and natural gas sector is one of the highest-emitting industrial sectors of methane and VOCs in the United States. In 2012, EPA issued a final rule setting standards of performance to limit emissions of VOCs from new, modified, and reconstructed sources in the oil and gas industry pursuant to CAA Section 111(b) (2012 Oil and Gas NSPSs). In an effort to achieve the Obama Administration's goal of reducing methane emissions from the oil and gas sector and to respond to petitions for reconsideration of the 2012 NSPS to address methane emissions, EPA proposed a rule revising and updating the 2012 NSPS in August 2015. After receiving over 900,000 comments, the agency issued the final rule on June 3, 2016 (2016 Oil and Gas NSPSs). EPA's final rule amended the 2012 Oil and Gas NSPSs, expanding the emission sources covered by the rule and establishing performance standards for GHGs (i.e., methane limitations) from a number of oil and gas emission sources. The final rule established, among other things, methane and VOC standards for emission sources and equipment not regulated under the 2012 Oil and Gas NSPSs , including hydraulically fractured oil well completions; pneumatic pumps; and fugitive emissions from well sites and compressor stations ; and methane emission standards for hydraulically fractured gas well completions and equipment leaks at natural gas processing plants that are currently regulated under the 2012 Oil and Gas NSPSs for VOC, but not for methane emissions. The final rule took effect on August 2, 2016, and applied only to equipment, processes, and activities in the production, processing, transmission, and storage phases of oil and natural gas systems constructed or modified after July 17, 2014; it did not apply to distribution entities. EPA noted that while the new methane standards cover many sources, those sources already complying with the 2012 Oil and Gas NSPSs would not likely be required to install additional emissions controls, as VOC controls also curb methane emissions. EPA estimated that the standards for new and modified sources are expected to reduce 510,000 short tons of methane in 2025, with estimated costs of $530 million and "climate benefits" of $690 million by 2025. As discussed above, the release of the final 2016 Oil and Gas NSPSs under CAA Section 111(b) triggered EPA's duty to issue Section 111(d) emission guidelines for existing oil and natural gas sources covered under the 111(b) rule. In 2016, EPA sent an Information Collection Request (ICR) to oil and natural gas companies seeking information on their existing oil and gas sources as a first step to regulating their methane emissions.