The Environmental Protection Agency (EPA) is finalizing amendments to the regulations that govern the processes and timelines for state and Federal plans to implement emission guidelines under Clean Air Act (CAA) New Source Performance Standards for existing sources (the "implementing regulations"). The amendments include revisions to the timing requirements for state and the EPA actions related to plans; the addition of mechanisms to improve flexibility and efficiency in plan processes; and new requirements for demonstration of timely meaningful engagement with pertinent stakeholders--including, but not limited to, industry, small businesses, and communities most affected by and vulnerable to the impacts of the plan. This action additionally provides a process for states' consideration of `remaining useful life and other factors' (RULOF) in applying a standard of performance; amends the definition of standard of performance in the implementing regulations; and clarifies compliance flexibilities that states may choose to incorporate into state plans, including trading or averaging. Finally, this action adds requirements for the electronic submission of state plans and provides several other clarifications and minor revisions to the implementing regulations.