Using a cluster-randomized controlled trial research design, this study investigates the effect of the first year of a three-year teacher professional development program on grades 1 and 2 student achievement in mathematics. Although the findings reported in the present report do not meet the standard cutoff for statistical significance (e.g., 95% confidence), effect-size estimates for student achievement in the first year of implementation were positive for the tests that focused on problem solving, applications of mathematics, and algebraic thinking, and they were negative for the computation-focused tests. Those effects varied by grade level. Implications and suggestions for next steps are discussed. [This report was prepared by the Florida Center for Research in Science, Technology, Engineering, and Mathematics (FCR-STEM), Learning Systems Institute, Florida State University.]