In the last two years, the U.S. Environmental Protection Agency (EPA) has issued a series of regulations targeting greenhouse gas (GHG) emissions under the Clean Air Act. SeePaulina Williams & Jenifer Sutter, “2010: EPA Completes its First Major Greenhouse Gas Rulemaking Initiatives,” Vinson & Elkins Climate Change Report (Feb. 21, 2011). Just as those regulations have been broad in scope, so are the legal challenges they now face. Business enterprises, trade associations, public policy groups, states, and local governments—joined by dozens of interveners and amici curiae—have filed a flood of petitions for judicial review, principally in the U.S. Court of Appeals for the D.C. Circuit. The litigation remains at an early stage, with the challengers’ initial merits briefs filed in March 2011, and the D.C. Circuit not likely to hear argument in the main body of challenges before the end of this year. But even a brief survey of the filings to date demonstrates the exceptional importance and complexity of these cases that bring before the courts what is among the most expensive and far-reaching environmental regulatory programs in U.S. history.