Teachers have an important influence on pupils' academic progress, yet the quality of teaching varies widely. Policymakers, school leaders, and teacher educators therefore face the challenge of designing and commissioning professional development (PD) to help all their teachers become as effective as the best teachers. In the last two decades, a large number of experimental evaluations have tested the impact of different approaches to teacher PD. However, impact varies widely, which raises the question of what--if anything--differentiates more effective PD from less effective PD. The objective of this review is to identify the characteristics of more effective PD. This report presents the results of an updated systematic review and meta-analysis, employing novel theory and methods to provide new insights on this important question. It includes the results of analyses pre-registered in the published protocol. In sum, the results suggest that policymakers, school leaders, and teacher educators should favour PD designs that incorporate more of the mechanisms set out in the theoretical framework. [For "Effective Professional Development. Guidance Report," see ED615913.]