In June 2014, the U.S. Environmental Protection Agency (EPA) proposed standards under §111(d) of the Clean Air Act (CAA)1 for state plans to reduce carbon dioxide emissions from existing fossil fuel-fired electric generating units (EGUs). These standards do not require states to use any particular means of emission reduction. However, their stringency depends in part on EPA’s conclusion that states could reduce these emissions not only by measures that directly reduce fossil fuel EGU emissions, but also by making the grid less dependent on fossil fuel through increased energy efficiency and increased reliance on nuclear and renewable generation. Some claim that this approach abandons a basic principle of CAA regulation without statutory warrant. This Comment argues the contrary. The rule could and probably would be implemented by long-established regulatory mechanisms that the U.S. Supreme Court has upheld. Moreover, EPA’s approach is neither unprecedented nor unlimited. Finally, the structure of the CAA sets limits in general and in particular on how far EPA could push this precedent.