In education systems around the world, planners and policy-makers are calling for more attention to whole-system improvement. The middle tier of education systems, composed of professionals working between the school and central levels, may offer a unique opportunity to facilitate collaboration, broker knowledge, scale innovations, and provide instructional direction to school-level practitioners. This research explores how middle-tier instructional leaders can form a nexus for improving the quality of education at scale. Through case studies it explores the professional practices and perceived impacts of instructional leaders and the enabling factors present in the systems where they work. It focuses on promising practices globally, and is designed to draw out insights and lessons for both policy-makers and practitioners.