The Clean Air Act (CAA) section 111(d) directs the EPA to promulgate regulations establishing a procedure “similar” to that provided by CAA section 110, under which states submit plans to implement the emission limitation requirements of CAA section 111 for existing sources. In 1975, EPA addressed this requirement by promulgating “implementing regulations” under 40 CFR part 60, Subpart B. These implementing regulations contain, among other things, deadlines for the submission of, and for EPA’s action on, state plans and deadlines for the promulgation of any federal plans, as well as regulations governing states’ consideration of existing sources’ remaining useful lives and other factors. In 2019, the EPA finalized 40 CFR part 60, Subpart Ba, a new subpart that updated the implementing regulations for prospective emission guidelines. However, in 2021, the U.S. Court of Appeals for the District of Columbia Circuit vacated certain provisions of the timelines for state and federal plans in Subpart Ba. EPA promulgated amendments to the timelines in Subpart Ba to address the court’s vacatur, as well as to make additional clarifications and add tools to aid states in developing and submitting state plans, on Nov. 17, 2023 (87 FR 79176). The EPA is now reassessing the 2023 amendments to Subpart Ba and intends to issue a proposed rule seeking public comment on any proposed changes.