A popular strategy for improving student achievement in math is to provide teachers with professional development (PD) that deepens their conceptual understanding of math. This report examines the effectiveness of such a PD program. The PD program included an 80-hour summer workshop (Intel Math) that focused on grades K–8 math, as well as 13 additional hours of collaborative meetings focused on analyzing student work and one-on-one coaching based on observations of teachers' lessons. More than 200 4th-grade teachers from six districts in five states were randomly assigned to either a treatment group that received the study PD or a control group that did not. The study PD had a positive impact on teachers' math knowledge and on their use and quality of mathematical explanations in class. However, the study PD did not have a positive impact on student achievement.