The Environmental Protection Agency (EPA) is approving elements of a State Implementation Plan (SIP) revision submitted by the State of Illinois regarding the infrastructure requirements of section 110 of the Clean Air Act (CAA) for the 2012 annual fine particulate matter (PM<INF>2.5</INF>) and 2015 ozone National Ambient Air Quality Standards (NAAQS). Further, EPA is approving the infrastructure requirements related to Prevention of Significant Deterioration (PSD) for previous NAAQS. The infrastructure requirements are designed to ensure that the structural components of each state's air quality management program are adequate to meet the state's responsibilities under the CAA. EPA received comments on its September 29, 2021, proposed rule and withdrew the accompanying Direct Final Rule (DFR). After considering the comments, EPA is approving the revisions to the Illinois SIP as requested by the State on September 29, 2017, May 16, 2019, and September 22, 2020.