Through their redundancy and the “reverse auction” dynamic they engender, competing class actions compromise the efficiency and fairness goals that justify the class action device and impose unnecessary costs on class members, defendants, the courts, and society at large. Yet, the Anti-Injunction Act, federalism, and comity concerns limit the ability of federal courts to enjoin competing state actions. Despite such limitations, some courts have utilized the “in aid of jurisdiction” exception to the Anti-Injunction Act to enjoin state actions that threaten to interfere substantially with the federal litigation. In this Note, Andrew Weinstein argues that building on these recent cases, federal courts should read the “in aid of jurisdiction” exception more expansively to permit injunctions in order to protect both the litigants and a court’s jurisdiction. Reconciling the merits of an injunction with the Anti-Injunction Act and related interests in federalism and comity, Weinstein devises four factors that federal courts should consider in determining whether to enjoin a competing state action.