The New Zealand Ministry of Education has published a best evidence synthesis iteration (BES) that identifies the characteristics of teacher professional development that make a positive difference for valued student outcomes. A companion best evidence synthesis iteration (BES) that identifies the leadership influences on valued student outcomes reveals that when school leaders promote and/or participate in effective teacher professional learning and development, this has more impact on student achievement than any other leadership activity. This article provides an overview of the findings about effective professional development and highlights the potential of such evidence to inform educational improvement. An example of a tool to support collaborative professional inquiry and knowledge building in schools is provided. The article highlights policy challenges for the systemic use of effective professional development and illustrates what is possible in two examples of high impact research and development (R & D) that have been effective across varied contexts. These examples illustrate the potential for educational improvement when professional learning is underpinned by cumulative high impact research and development in education. (Contains 3 tables and 3 figures.)