Analyzes the participation of SMEs in production networks, determinants, and policy implications using a data set covering 5,900 firms in five ASEAN economies at different stages of development, notably Indonesia, Malaysia, the Philippines, Thailand, and Vietnam. Concludes that SMEs have modestly increased their production network participation since the late 1990s, although large firms still dominate. Suggests that determinants are a supportive business environment as well as firm-specific factors such as size, foreign ownership, human capital, technology, age, and access to credit. Recommends tackling residual supply side and policy constraints in order to increase SME participation. Includes graphic illustrations of the share of SME and large firm exports in total exports, and of the share of the top 25 percent SME exporters in each country. Lists results of the T-test on key variables for SMEs in production networks vs SMEs outside production networks.