The theory of production networks is incomplete, and it does not scale up to industry- or macro-level in a convincing way. Moreover, production networks are not fully observed in official figures such as international trade statistics. Because firm heterogeneity as well as the heterogeneity of inter-firm relationships is at the center of mechanism, micro panel data analysis is certainly useful. However, such data rarely tell us much about market structure, competition, and inter-firm relationships. We researchers are just beginning to form a broad consensus on the nature and characteristics of production networks.