The U.S. Environmental Protection Agency (EPA) is revising its response to the U.S. Supreme Court decision in Michigan v. EPA, which held that the EPA erred by not considering cost in its determination that regulation under section 112 of the Clean Air Act (CAA) of hazardous air pollutant (HAP) emissions from coal- and oil-fired electric utility steam generating units (EGUs) is appropriate and necessary. After primarily comparing the cost of compliance relative to the benefits of HAP emission reduction from regulation, the EPA finds that it is not “appropriate and necessary” to regulate HAP emissions from coal- and oil-fired EGUs, thereby reversing the Agency’s previous conclusion under CAA section 112(n)(1)(A) and correcting flaws in the Agency’s prior response to Michigan v. EPA. We further find that finalizing this new response to Michigan v. EPA will not remove the Coal- and Oil-Fired EGU source category from the CAA section 112(c) list of sources that must be regulated under CAA section 112(d) and will not affect the existing CAA section 112(d) emissions standards that regulate HAP emissions from coal- and oil-fired EGUs. The EPA is also finalizing the residual risk and technology review (RTR) conducted for the Coal- and Oil-Fired EGU source category regulated under national emission standards for hazardous air pollutants (NESHAP), commonly referred to as the Mercury and Air Toxics Standards (MATS). Based on the results of the RTR analyses, the Agency is not promulgating any revisions to the MATS rule.