On February 16, 2012, EPA promulgated National Emission Standards for Hazardous Air Pollutants for Coal- and Oil-fired Electric Utility Steam Generating Units (77 FR 9304). The rule (40 CFR part 63, subpart UUUUU), commonly referred to as the Mercury and Air Toxics Standards (MATS), includes standards to control hazardous air pollutant emissions from new and existing coal- and oil-fired electric utility steam generating units located at both major and area sources of hazardous air pollutant emissions. This entry in the regulatory agenda describes EPA’s review of this action pursuant to section 610 of the Regulatory Flexibility Act (5 U.S.C. 610) to determine if the provisions that could affect small entities should be continued without change or should be rescinded or amended to minimize adverse economic impacts on small entities. As part of this review, EPA is considering comments on the following factors: (1) The continued need for the rule; (2) the nature of complaints or comments received concerning the rule; (3) the complexity of the rule; (4) the extent to which the rule overlaps, duplicates, or conflicts with other Federal, State, or local government rules; and (5) the degree to which the technology, economic conditions or other factors have changed in the area affected by the rule. The results of EPA’s review will be summarized in a report and placed in the docket at the conclusion of this review. This review’s Docket ID number is EPA-HQ-OAR-2021-0152.