With decades of evidence to support early childhood development (ECD) programs and policies, investment in ECD has expanded worldwide. Currently, over 70 nations have national ECD legislation, the majority in the last 20 years. However, with these increased investments comes evidence that the capacity of policy systems to support ECD--across health, education, social protection, and other sectors--is weak, with unfulfilled developmental potential a serious consequence within and across countries. This report aims to develop a research agenda on the systems-level factors--at national, subnational, and local or municipal levels--that may enable or constrain program site-level implementation. Two types of scale--"small to bigger" and "big to better"--are described, as well as the specific challenges of these processes in the field of ECD. Systems factors are reviewed at the three levels, with implications of each for measurement. Finally, methodological challenges and directions are discussed with the aim of informing a research agenda to support national policy progress in early childhood development. [Work on this paper was supported by funding from the NYU Abu Dhabi Research Institute to the Global TIES for Children Center at New York University.]