In an increasingly globalised world，disputes with cross-border elements are increasingly likely，and private international law is thus a subject of growing importance. At the same time，however，it is a subject whose theoretical foundations remain contested and insufficiently developed. This paper argues that private international law should be viewed as giving effect to an underlying principle of “justice pluralism”—the acceptance of diversity in the private legal orders of different systems. The rules of private international law serve the purpose of structuring and ordering that diversity-they are part of the architecture of the international system. This argument is developed based on an analysis of the historical origins and evolution of private international law. The idea of understanding private international law from a systemic perspective is explored further through examination of the role of private international law within federal systems，in particular the European Union，Canada and Australia. Within these systems，there is an emerging understanding of the connection between private international law and constitutional law，that private international law is part of the horizontal ordering of legal authority between federal states. The paper argues that this federal understanding should inform our ideas of private international law at the global level，as part of the constitutionalisation of the international legal order. Private international law can thus be viewed as an important counterpart and counter-balance to public international law-while public international law aspires to embody universal rules and principles，private international law facilitates the coexistence and tolerance of global diversity.