The U.S. Environmental Protection Agency (EPA) is taking this action to address interstate transport of ozone pollution under the "good neighbor provision" of the Clean Air Act (CAA). This final action is taken in response to the United States Court of Appeals for the District of Columbia Circuit's (D.C. Circuit) remand of the Cross- State Air Pollution Rule (CSAPR) Update in Wisconsin v. EPA on September 13, 2019. The CSAPR Update finalized Federal Implementation Plans (FIPs) for 22 states to address their good neighbor obligations for the 2008 ozone National Ambient Air Quality Standards (NAAQS). The D.C. Circuit found that the CSAPR Update, which was published on October 26, 2016 as a partial remedy to address upwind states' obligations prior to the 2018 Moderate area attainment date under the 2008 ozone NAAQS, was unlawful to the extent it allowed those states to continue their significant contributions to downwind ozone problems beyond the statutory dates by which downwind states must demonstrate their attainment of the air quality standards. On the same grounds, the D.C. Circuit also vacated the CSAPR Close-Out in New York v. EPA on October 1, 2019. This final rule resolves 21 states' outstanding interstate ozone transport obligations with respect to the 2008 ozone NAAQS. This action finds that for 9 of the 21 states for which the CSAPR Update was found to be only a partial remedy (Alabama, Arkansas, Iowa, Kansas, Mississippi, Missouri, Oklahoma, Texas, and Wisconsin), their projected ozone precursor emissions in the 2021 ozone season and thereafter do not significantly contribute to a continuing downwind nonattainment and/or maintenance problem, and therefore the states' CSAPR Update FIPs (or the SIPs subsequently approved to replace certain states' CSAPR Update FIPs) fully address their interstate ozone transport obligations with respect to the 2008 ozone NAAQS. This action also finds that for the 12 remaining states (Illinois, Indiana, Kentucky, Louisiana,