Purpose: A growing trend in instructional improvement efforts is the use of formative assessment informed by research-based developmental trajectories of how students gain deeper understanding of subject matter content over time. This article reports the findings of a large-scale experimental study of an innovative mathematics professional development program called the Ongoing Assessment Project (OGAP) in a large urban school district that is based on the theory of learning trajectory-oriented formative assessment. Research Methods: This research employs a randomized controlled trial and structural equation modeling. Findings: The study provides causal evidence of the impact of OGAP on teachers' knowledge of student thinking and student learning outcomes. The results further demonstrate that OGAP professional development and ongoing supports significantly enhanced teachers' knowledge of formative assessment and improved grades 3-5 student outcomes on the Pennsylvania State high-stakes standardized tests in mathematics. Structural equation modeling of within-treatment variation demonstrates the relationship between teacher implementation and student performance, identifies both individual teacher characteristics and program supports related to implementation and performance, and raises additional areas for further investigation. Implications: Although formative assessment has long been viewed as a potentially powerful way to provide teachers with feedback on student understanding, the integration of learning trajectories helps address the "Now what?" question by providing teachers with more specific and actionable guidance to make more informed instructional responses. This study joins the small set of causal research on effective mathematics professional development programs that utilize learning trajectory-oriented formative assessment that point the way to promising pathways to improved student performance.