We present results from a meta-analysis of 95 experimental and quasi-experimental pre-K-12 science, technology, engineering, and mathematics (STEM) professional development and curriculum programs, seeking to understand what content, activities, and formats relate to stronger student outcomes. Across rigorously conducted studies, we found an average weighted impact estimate of +0.21 standard deviations. Programs saw stronger outcomes when they helped teachers learn to use curriculum materials; focused on improving teachers' content knowledge, pedagogical content knowledge, and/or understanding of how students learn; incorporated summer workshops; and included teacher meetings to troubleshoot and discuss classroom implementation. We discuss implications for policy and practice.