This chapter focuses on one author of cross-fertilization, the Permanent Court of Arbitration (PCA). The PCA plays various roles in the development of international law. In recent decades the PCA has seen significant procedural cross-fertilization among international institutions, for example with the UNCITRAL Arbitration Rules informing the PCA’s own modern rules of procedure. PCA-administered tribunals have engaged in substantive cross-fertilization, citing ICJ decisions as well as awards issued by PCA- and ICSID-administered tribunals, among others. There are limits to the PCA’s role in cross-pollination and convergence, particularly the principle of party autonomy and and excess of mandate by the tribunal being grounds for an award to be set aside. Party-driven limits to convergence are exemplified in the high degree of variation in transparency among PCA-administered arbitrations, with proceedings ranging from the highly transparent to the fully confidential. Despite these limits, this chapter concludes that both as an actor and as a framework, the PCA serves as a mechanism whereby procedural law and substantive international law are distilled, developed, and diffused.