On April 29, 2014, the Supreme Court reversed the D.C. Circuit’s 2012 vacatur of the Cross-State Air Pollution Rule (CSAPR; also referred to as the Transport Rule). Overall, the Court disagreed with the D.C. Circuit’s decision that EPA was required to specifically and proportionally determine each state’s contribution to another downwind state’s impairment of air quality. The Supreme Court also disagreed that EPA was required to allow states the opportunity to design state implementation plans (SIPs) to address the identified contributions before implementing CSAPR. Thus, the Court reversed and remanded the decision to the D.C. Circuit for further proceedings consistent with the opinion, including issues that the D.C. Circuit has not yet addressed. The Clean Air Interstate Rule (CAIR) remains in place until the D.C. Circuit lifts its vacatur of the rule, and it is currently unclear what steps EPA will take following that action.