To improve teaching and learning outcomes, teachers and head teachers need whole-system support to build professionalism and enhance their practice. Roles at the middle tier of education systems, or those professionals working between the school and central level, offer unique capacities to facilitate collaboration, broker knowledge, scale innovations, and provide instructional direction to school-level practitioners. This research explores how middle-tier instructional leaders can become a nexus for change to improve the quality of education. It focuses on highlighting promising practices globally and is designed to draw out insights and lessons for both policy-makers and practitioners. Case studies were conducted in five jurisdictions -- Delhi (India), Jordan, Rwanda, Shanghai (China), and Wales -- where new roles have been created or existing roles re-oriented towards coaching and support. The studies explored the professional practices and perceived impacts of instructional leaders, and the enabling factors present in the systems in which they work. This brief is part one of a three-part series focusing on what the middle tier is and why it is important for improving learning. Brief 2 presents the professional practices and perceived impacts of instructional leaders, while Brief 3 presents the factors enabling them to become change agents. [For brief 2, see ED629429. For brief 3, see ED629428.]