The multilateral trade regime has significantly evolved in both depth of policy commitments among WTO members and breadth of membership. As members deepened policy commitments toward each other, total membership simultaneously expanded from 23 countries in 1947 to 159 countries in 2013. This chapter reviews why nonmembers seek to join the trade regime and explains the flexible accession process that governs membership expansion. A case study of Mexico’s experience joining GATT highlights the economic and political considerations in membership decisions. Economic reform and foreign policy alignment are key variables to account for when countries enter the regime. The chapter emphasizes that theories seeking to explain the selection of members must guide studies that use membership status as an identification strategy to estimate the trade regime’s effects on economic outcomes.