Policy makers increasingly include provisions aimed at fostering professional community as part of reform initiatives. Yet little is known about the impact of policy on teachers' professional relations in schools. Drawing theoretically from social capital theory and methodologically from qualitative social network analysis, this article explores how district policies influence teachers' social networks in eight elementary schools in two districts involved in the scale-up of mathematics curriculum. It is argued that policy affects whom teachers seek out for discussion of mathematics instruction but that differences in policy provisions lead to variations in the nature and quality of interactions. Furthermore, school leaders mediate district policy, thereby influencing these patterns of interaction. By uncovering the dynamics by which policy influences teachers' social networks, this article contributes to understandings of the factors that foster the development of social capital. It also uncovers opportunities for intervention for those designing policy initiatives to support implementation of instructional innovations. (Contains 2 figures, 5 tables, and 17 notes.)