Hundreds of millions of dollars every year are spent by states and school districts on providing preK-12 teachers with professional development to improve their mathematics and science instruction, but these efforts are not always successful in resulting in better instruction for the nation's students. Research on teacher professional learning over the last several decades has expanded the field's understanding of how and under what conditions teachers learn to teach mathematics and science, and this body of work can inform these efforts. Findings and insights about teacher professional learning across this body of work, when understood collectively, can be powerful towards supporting teacher change. This project synthesizes research on teacher learning to distill ideas and develop a new, deeper understanding of how preK-12 teacher professional learning in mathematics and science influences teacher beliefs, knowledge, and practice. This study will provide information that enables states, districts, and schools to elevate the quality of teacher professional learning in STEM to lead to more effective instruction that fosters more and better STEM student engagement and learning and motivates more students to choose STEM careers. This project's potential benefit to society is to increase the field's capacity to improve STEM teacher professional learning in ways that transform and strengthen teacher effectiveness, which in turn improve STEM engagement and learning for all stud