In June 2014, EPA proposed an emission guideline for emissions of carbon dioxide (CO2) from existing fossil fuel-fired electric generating units (EGUs). This rulemaking--commonly known as the Clean Power Plan (CPP)--identifies as the “best system of emission reduction” a set of four “building blocks” that electric utilities can undertake or purchase credits for to reduce emissions from regulated EGUs. This Comment considers whether the building block approach is permissible under CAA §111(d).