Using implementation research to understand lessons in reducing child mortality.

Under-5 mortality decreased significantly worldwide between 2000 and 2015, but there is still progress to be made, particularly in lower- and middle-income countries. This supplement shares the work over the last four years on a project to understand how six countries (Bangladesh, Ethiopia, Nepal, Peru, Rwanda, and Senegal) were more successful in decreasing child mortality than many of their regional and economic peers. The use of implementation research across these countries identifies common implementation strategies and contextual factors that can facilitate or impede successful implementation of an evidence-based intervention and explores a common pathway to implementation. The work highlights how the use of implementation research to understand the "how" and the "why" behind countries' success provides important actionable knowledge and lessons to country-level decision-makers, donors, and implementers as we arrive at the midpoint of the Sustainable Development Goal era.