The minute-to-minute and day-by-day use of assessment for learning holds great potential to change the trajectory of student learning in U.S. classrooms. But without effective and scalable systems of professional development that actually lead teachers to adopt these practices, the utility and impact of assessment for learning will be quite limited. Drawing on learning theory, expertise research, and research on effective professional development, this paper presents the theoretical and empirical basis behind an evolving program of professional development that employs school-embedded teacher learning communities as a central component, called "Keeping Learning on Track[R]." Implementations of the "Keeping Learning on Track" program in two districts and a consortium of districts are described in detail. (Contains 2 notes, 2 figures and 1 table.)