On January 2, 2011, EPA’s much-anticipated prevention of significant deterioration and Title V Greenhouse Gas Tailoring Rule took effect, expanding the reach of the Clean Air Act and creating a phased-in approach to greenhouse gas regulation that initially targets the nation’s largest emitters but will gradually encompass additional sources. Numerous challenges threaten the rule’s long-term viability, including a regulatory alternative that could gain traction in the continued absence of a legislative response to the issue of climate change.