Agreements about foreign trade policy have long been key features of the international political economy. Among the most important agreements of this sort are preferential trade agreements (PTAs), which are designed to foster economic integration among member-states by improving and stabilizing the access that each member has to the other participants’ markets. PTAs are a broad class of international agreements that include common markets, customs unions (CUs), free trade areas (FTAs), and economic unions. These agreements have marked the global landscape for centuries, but they have proliferated especially rapidly over the past half-century, and hundreds of them currently dot the international system. The spread of PTAs continues unabated and is one of the most significant trends in the international political economy