While school readiness is gaining currency around the globe, there remain many issues linked with a cohesive understanding of the concept and its applications to improve the learning and development of all children, the quality of schools, and the participation of families and communities. The aim of this paper is to provide the latest evidence and knowledge on school readiness within an easily understandable framework relevant to the lives of young children in the majority of the world. To that end, this paper focuses on three basic yet critical questions: What is school readiness? Why is school readiness important? And what are the consequences of inaction? This paper presents a broad concept of school readiness, describing in detail three dimensions: children's readiness for school; schools' readiness for children; and families' and communities' readiness for school. It then proceeds to provide a rationale for the importance of school readiness, not just for individual children, but also for societal and national development more generally. Finally, the paper makes a strong case regarding the costs of inaction for children, families, communities and countries, and addresses international strategies for action. (author abstract)