The Sustainable Development Goal's "Education 2030" agenda includes an explicit focus on early childhood development. Target 4.2 states that all children are "developmentally on track" at the start of school. What does it mean for a child to be developmentally on track, and how should it be measured, especially in an international context? In this article, principles of child development with implications for measurement are described, together with issues in accurately capturing the complex nature of early development with feasible, cost-effective measures. Three measures are described, with an emphasis on identifying the policy relevance, feasibility, and methodologies that influence their potential usefulness for measuring progress towards global education goals. Directions for measuring early childhood development and learning are outlined.