By illuminating why and how interventions work in real world settings, Implementation Research (IR) is a powerful tool for increasing the likelihood that evidence-based interventions, programmes and policies are successfully implemented. The insights that IR generates help bridge the 'know-do gap'--the gap between what we know works and what actually happens on the ground when we try to put a policy or intervention into place. IR is a means for increasing the likelihood of successful outcomes, reducing the risk of wastage and failure and accelerating programme and system improvements to reduce inequities and achieve desired results. This paper, prepared by the Centre for Evidence and Implementation in collaboration with UNICEF, aims to promote a shared understanding of IR and its relevance to UNICEF's work. [This paper was prepared by the Centre for Evidence and Implementation, Norwegian University of Science and Technology (NTNU), and CHAIN--Centre for Global Health Inequalities Research.]