On April 2, 2007, a unanimous U.S. Supreme Court issued its much anticipated decision in Environmental Defense v. Duke Energy Corp., No. 05-848 (Apr. 2, 2007). In vacating a decision by the Fourth Circuit in Duke Energy’s favor, the Court settled a split between the Fourth and D.C. Circuits concerning the reach of “modification” under the Prevention of Significant Deterioration program of the Clean Air Act.