More than half of U.S. children fail to meet proficiency standards in mathematics and science in fourth grade. Teacher professional development and curriculum improvement are two of the primary levers that school leaders and policymakers use to improve children's science, technology, engineering and mathematics (STEM) learning, yet until recently, the evidence base for understanding their effectiveness was relatively thin. In recent years, a wealth of rigorous new studies using experimental designs have investigated whether and how STEM instructional improvement programs work. This article highlights contemporary research on how to improve classroom instruction and subsequent student learning in STEM. Instructional improvement programs that feature curriculum integration, teacher collaboration, content knowledge, pedagogical content knowledge, and how students learn all link to stronger student achievement outcomes. We discuss implications for policy and practice.