For decades, the class action has been in the crosshairs of defense-side procedural warfare. Repeated attacks on the class action by the defense bar, the U.S. Chamber of Commerce, and other defense-side interest groups have been overwhelmingly successful. None was more successful than the culmination in the late aughts of the “arbitration revolution”—a forty-year campaign to eliminate class actions through forced arbitration provisions in private contracts. The effects for civil justice were profound: The arbitration revolution all but erased scores of civil rights claims, wage-theft claims, claims for workplace sexual harassment, and claims for consumer fraud from the civil justice landscape. The effects for social and racial justice were likewise profound: The arbitration revolution has systematically and disproportionately foreclosed the legal rights of minorities, women, wage-and-hour workers, and the working poor.Yet now, just when one would expect the defense bar to be taking a victory lap, the entire forty-year campaign to enfeeble and ultimately eliminate the class action through arbitration is being abandoned. The same corporate entities that waged, and won, the decades-long war are now retreating from arbitration and seeking refuge in class actions. The reason? An unforeseen and largely inconceivable counter-offensive by a small subset of the plaintiffs’ bar—Mass Arbitration.Mass Arbitration has single-handedly upended the arbitration revolution. Yet no scholarly attention has been given to the phenomenon. This Article presents a foundational analysis of the subject.This Article develops the first and only case study of Mass Arbitration and taxonomizes the results. What emerges is not a variation on old themes, but, instead, a new and distinct model of aggregate dispute resolution. The investigation reveals important differences between the mass arbitration model and existing forms of aggregate dispute resolution, exposes untheorized elements of existing models, recasts long-standing debates in complex litigation theory and jurisprudence, and provides new perspectives on private procedural ordering as well as the relationship between private procedural design and social justice. Mass arbitration, in other words, is a phenomenon in its own right. More importantly, it offers a window into the future of civil justice