This article describes an analytic approach for situating teachers' instructional practices within the institutional settings of the schools and school districts in which they work. The approach treats instructional leadership and teaching as distributed activities and involves first delineating the communities of practice within a school or district whose enterprises are concerned with teaching and learning and then analyzing three types of interconnections between them: boundary encounters, brokers, and boundary objects. We illustrate the analytic approach by focusing on one urban school district in which we have conducted an ongoing collaboration with a group of middle school teachers. In doing so, we clarify the critical role that school and district-level leaders can play in mediating state and federal high-stakes accountability policies. We conclude by discussing the implications of the analysis for the process of upscaling and the diffusion of instructional innovations. (Contains 6 footnotes.)