Teacher professional learning is a critical part of the mathematics education landscape. For decades, professional learning has been the primary strategy for developing the skills of the teaching workforce and changing how teachers interact with students in classrooms around academic content. Professional learning also can be expensive for districts, both financially and in terms of teacher time. Given these investments, most school leaders wish to spend their professional development dollars efficiently, making decisions about professional learning design that maximize teacher and student learning. However, despite more than two decades of rigorous research on professional learning programs, practitioners have little causal evidence on which professional learning design features work to accelerate teacher learning. This project seeks to identify features of teacher professional learning experiences that lead to better mathematics outcomes for both teachers and students.