"Millions Learning: Scaling up Quality Education in Developing Countries" tells the story of where and how quality education has scaled in low- and middle-income countries. The story emerges from wide-ranging research on scaling and learning, including 14 in-depth case studies from around the globe. Ultimately, "Millions Learning" finds that from the slums of New Delhi to the rainforest in Brazil, transformational change in children's learning is happening at large scale in many places around the world. The authors find that successful scaling of quality learning often occurs when new approaches and ideas are allowed to develop and grow on the margins and then spread to reach many more children and youth. In "Millions Learning," the authors identify 14 core ingredients, in different combinations depending on the context, that contribute to scaling quality learning. Each of these ingredients is central for scaling effective approaches that improve learning. Their importance is frequently reinforced from evidence in the broader scaling literature. They include essential elements for designing, delivering, financing, and enabling the scaling of quality education. The authors recommend the following five main actions to create inclusive and adaptive education ecosystems and ultimately help scale quality learning for millions of children and youth: (1) Develop a culture of Research & Development in Education; (2) Share new ideas through a network of idea hubs; (3) Activate talent and expertise outside the classroom; (4) Fund the middle phase; and (5) Measure and learn what works through better learning and scaling data. Three annexes present: (1) "Millions Learning" case study selection process; (2) 14 "Millions Learning" case studies; and (3) Acknowledgements. A bibliography is included. [This report was written with Eileen McGivney.]