Available Open Access under CC-BY-NC licence. What matters most in how poverty shapes children's wellbeing and development? How can data inform social policy and practice approaches to improving the outcomes for poorer children? Using life course analysis from the Young Lives study of 12,000 children growing up in Ethiopia, India, Peru and Vietnam over the past 15 years, this book draws on evidence on two cohorts of children, from 1 to 15 and from 8 to 22. It examines how poverty affects children's development in low and middle income countries, and how policy has been used to improve their lives, then goes on to show when key developmental differences occur. It uses new evidence to develop a framework of what matters most and when and outlines effective policy approaches to inform the no-one left behind Sustainable Development Goal agenda.