On April 16, 2020, the U.S. Environmental Protection Agency (EPA) released a prepublication final rule (along with a fact sheet and memorandum) revising its supplemental finding for the Mercury and Air Toxics Standards (MATS) and finalizing the results of a residual risk and technology review of the MATS. The revised supplemental finding is in response to the 2015 U.S. Supreme Court decision in Michigan v. EPA, which held that EPA failed to consider costs when determining whether it is “appropriate and necessary” to regulate hazardous air pollutants (HAPs) from coal- and oil-fired electric utility steam generating units (EGUs). In response to the decision, EPA made a supplemental finding in 2016. The revised supplemental finding concludes that the 2016 supplemental finding improperly considered co-benefits from non-HAP emissions reductions as part of the cost-benefit analysis. While the revised finding concludes based on a recalculated cost-benefit analysis that it is not “appropriate and necessary” to regulate HAPs from coal- and oil-fired EGUs, it does not remove these stationary sources from the list of sources that must be regulated under Section 112 of the Clean Air Act. Accordingly, the rule also provides final action on a residual risk and technology review required under Section 112 that shows that residual risk from HAP emissions are at acceptable levels and there is no need for changes to the MATS rule. While keeping the MATS rule in place, the final rule introduces an approach to cost-benefit analysis that places much less weight on co-benefits and could be used in subsequent rulemakings.