Editors' Summary: The recent Supreme Court decision in Massachusetts v. EPA is predicted to have significant impacts on standing to sue, regulation pursuant to the CAA, and pending climate change cases. Arnold W. Reitze examines the Massachusetts v. EPA decision and its potential implications. In this Article, he describes the history of the litigation, the majority opinion and dissents from the Supreme Court Justices, and the decisions that EPA and Congress now face in light of this decision.