The global learning crisis is driving a focus on improving learning. Teacher development and improving teaching quality, is therefore at the heart of many education systems’ policies and programs. The Teacher Development Multi-Year Studies were initiated by the Australian Government’s Department of Foreign Affairs and Trade (DFAT) to provide evidence on the impact of its investments in teacher development and how they can be more effective. Over 5 years, the Australian Council for Educational Research (ACER) conducted research in 3 countries: Timor-Leste, Vanuatu and Lao People’s Democratic Republic (PDR). The research investigated the extent to which Australian investments in the professional development of teachers have contributed to improved outcomes. It also sought to understand the conditions of success for teacher professional development investments, and how lessons learned can inform future programming. This report highlights how teacher professional learning can work with system and contextual factors to lead to better outcomes. It provides a set of strategies for future thinking related to teacher development investments.