This proposed rulemaking would update the final State Implementation Plan (SIP) Requirements Rule for the 2008 Ozone National Ambient Air Quality Standards (NAAQS) (80 FR 12264, March 6, 2015) to reconcile regulatory provisions that were vacated as part of the decision in South Coast Air Quality Management District v. EPA, 882 F.3d 1138 (D.C. Cir. 2018) (South Coast II) with those listed in part 51 of the Code of Federal Regulations. The 2008 SIP Requirements Rule governs attainment planning requirements that apply to areas designated nonattainment for the 2008 ozone NAAQS, and states in the Ozone Transport Region, as well as anti-backsliding requirements for areas once designated nonattainment for the revoked ozone NAAQS. This proposed action would clarify national policy by updating affected provisions in the 2008 ozone SIP Requirements Rule to reflect the outcome of South Coast II and ensure that states understand the requirements that apply to them for continued implementation of the ozone NAAQS.