The OGAP intervention incorporates two approaches to mathematics instruction that are well supported by research, but have not been uniformly adopted in U.S. schools: The first is ongoing formative assessment by teachers to tailor instruction to student needs (Black & Wiliam, 1998); the second is the use of learning trajectories to specify conceptual pathways for student development within specific domains (Daro et al., 2011; Sztajn et al., 2012). Implementing OGAP in schools involves a great deal of learning on the part of teachers and school leaders. It also involves embracing a fundamental shift in how one thinks about learning and designing instruction. The OGAP intervention provided grade 3-5 teachers with tools to support learning-trajectory formative assessment practices (described in other papers) and professional development (week- long summer training and several additional training days during the academic year). We also included a site-based approach to increase understanding and capacity across all participating teachers. Each school was asked to hold a bimonthly Professional Learning Community (PLC) with the primary purpose of collaboratively analyzing student work, using learning trajectory frameworks and determining appropriate instructional responses. PLCs were envisioned as a primary structure to support use of OGAP in the schools throughout the year. They were also intended to situate the use of OGAP tools and routines in each school and normalize opportunities for discourse about student thinking among teachers (Putnam & Borko, 2000). By providing teachers with ongoing and consistent opportunities to discuss their own students’ work and use the OGAP frameworks to make instructional decisions, we anticipated they could potentially deepen teachers’ understanding of OGAP and their own students’ thinking. In this paper, we examine five PLCs in the OGAP project in order to consider the extent to which this potential was realized.