Given the size and scope of the teacher workforce, supporting the development of instructional effectiveness among teachers currently in schools is necessary to remediate educational inequalities and improve outcomes for students. Schools and school systems recognize this need and spend vast sums on professional development opportunities and training efforts. However, these efforts rarely lead to improved instructional practice at scale. This strand will explore how educators improve their knowledge, internal capacities, and pedagogical skills, and how state and district policies can best support development at scale. It will attend to development throughout the career, exploring connections between pre-service teacher preparation and in-service professional development. It will also attend to the training and development of school leaders.