Math for All (MFA) is an intensive professional development (PD) program for in-service teachers. It consists of five one-day workshops and classroom-based assignments, providing a total of 40 hours of PD. MFA engages teams of general and special education teachers in adapting math lessons collaboratively to help all students, including those with disabilities, achieve high-quality learning outcomes in mathematics. A cluster randomized controlled trial (RCT) was used to test the efficacy of MFA; it included 32 schools, 98 4th and 5th grade teachers, and approximately 1,500 4th and 5th grade students. MFA had statistically significant, positive effects on teachers' self-reports of their preparedness and comfort with teaching mathematics to students with disabilities. A school-level analysis found a moderate MFA effect on student achievement. Quasi-experimental analyses of a subgroup of teachers showed initial evidence of MFA impacts on their classroom practices.