The intersection of federalism and class-action litigation has been an area of significant controversy in recent years. With the 2005 Class Action Fairness Act placing more high-stakes class actions into federal court, an especially crucial question is the extent to which the Erie doctrine and the Rules Enabling Act (REA) require federal courts to follow state class-action law. The Supreme Court’s decision in Shady Grove Orthopedic Associates v. Allstate Insurance Co. begins to confront this issue, but many unanswered questions remain. Under several lines of argument that were neither made nor considered in Shady Grove, the Erie doctrine and the REA would require federal courts to apply state class-action law, whether state law is more tolerant or less tolerant of class actions than the prevailing federal approach. In particular, Shady Grove leaves open the possibility that state class-action law may influence a federal court’s application of Rule 23’s certification requirements, as well as other matters - available remedies, statutes of limitations, and the preclusive effect of the ultimate judgment - that can arise during the course of a putative class action. This article identifies these important unresolved issues, and then suggests a more precise doctrinal framework for addressing them (and Erie/REA issues more generally).