The authors describe an impact study of Classroom Assessment for Student Learning (CASL), a widely used professional development program in classroom and formative assessment. Researchers randomly assigned 67 elementary schools to receive CASL materials or continue with regularly scheduled professional development. Teachers in CASL schools formed learning teams, trained with CASL materials, and implemented CASL in their classrooms under real-world conditions and without any involvement of, or requirements from, the researchers. Analysis of all schools and 9,596 students failed to yield statistically significant impacts of CASL on student mathematics achievement as measured by the statewide test. Impact analyses with 231 teachers yielded statistically significant positive impacts of CASL on teacher knowledge of assessment and the frequency of student involvement in classroom assessment. No statistically significant impacts were found on teachers' assessment practice. CASL implementation fidelity was below the CASL developer recommendations. Findings suggest potential of CASL for improving teacher outcomes.