This introduction reviews scholarship on international legal fragmentation, lays out a framework for understanding international judicial cross-fertilization, and previews the contributions and their findings. Existing scholarship on international legal fragmentation, we argue, has moved through three phases over the past several decades. In the first, legal scholars and practitioners reacted with alarm to the judicial proliferation of the post–Cold War years, which they feared would create overlapping jurisdiction and conflicting interpretations of law. Following this period, the new century saw the pendulum swing toward a second, more optimistic picture in which international courts addressed fragmentation through “management” techniques, producing unity in international law. We can detect the opening salvos of a third wave, as skeptics have questioned the management account, pointing to the mixed motives of international judges and the limits of cross-fertilization. In this volume, we build on the existing literature by theorizing the actors of cross-fertilization and their motives, and by distinguishing between procedural and substantive cross-fertilization.