The American Institutes for Research and partners are conducting a large-scale study of the impact of teacher professional development in mathematics. The Mathematics Professional Development (PD) Impact Study is a randomized controlled trial designed to test the impact of a scalable program of professional development that holds promise for improving mathematics instruction and student achievement at the seventh grade. The professional development program targeted knowledge and teaching of rational numbers topics – i.e., fractions, decimals, percent, ratio and proportion. These topics are among the most difficult for young students to grasp and are considered an essential foundation for algebra.