The Environmental Protection Agency (EPA) is finalizing nonattainment area and ozone transport region (OTR) implementation requirements for the 2015 ozone national ambient air quality standards (NAAQS) (2015 ozone NAAQS) that were promulgated on October 1, 2015. This final rule is largely an update to the implementing regulations previously promulgated for the 2008 ozone NAAQS, and we are retaining without significant revision the majority of those provisions to implement the 2015 ozone NAAQS. This final rule addresses a range of nonattainment area and OTR state implementation plan (SIP) requirements for the 2015 ozone NAAQS, including attainment demonstrations, reasonable further progress (RFP) and associated milestone demonstrations, reasonably available control technology (RACT), reasonably available control measures (RACM), major nonattainment new source review, emissions inventories, the timing of required SIP submissions and compliance with emission control measures in the SIP. The EPA is not taking any final action regarding our proposed approach for revoking a prior ozone NAAQS and establishing anti-backsliding requirements; the agency intends to address any revocation of the 2008 ozone NAAQS and any potential anti-backsliding requirements in a separate future rulemaking.