This article describes the impact study of Classroom Assessment for Student Learning (CASL), a widely-used professional development program in classroom and formative assessment. Teachers in 33 schools randomly assigned to use CASL formed learning teams, trained with CASL materials, and implemented CASL in their classrooms. The teachers in the 34 schools randomly assigned to the control group conducted business as usual. After two years of implementation, CASL had positive impacts teachers’ knowledge of assessment and the frequency of student involvement in classroom assessment. No statistically significant impacts were found on teachers’ assessment practice or on student achievement.