In this final action, the U.S. Environmental Protection Agency (EPA) is finalizing a significant contribution finding (SCF) for purposes of regulating source categories for greenhouse gas (GHG) emissions, under section 111(b) of the Clean Air Act (CAA) for electric generating units (EGUs), and in doing so, reaffirming that EGUs remain a listed source category. The EPA has reached that conclusion by articulating a framework under which source categories are considered to contribute significantly to dangerous air pollution due to their GHG emissions if the amount of those emissions exceeds 3 percent of total U.S. GHG emissions. The EPA is applying the 3-percent threshold to the EGU source category to demonstrate that GHG emissions from the EGU source category would contribute significantly to dangerous air pollution. While EGU GHG emissions exceed this threshold by a sufficient magnitude to warrant an SCF without more ado, the EPA has also, for completeness, analyzed EGU emissions under a secondary criteria framework, which also demonstrates the propriety of the SCF.