Children's understanding of fractions, including their symbols, concepts, and arithmetic procedures, is an important facet of both developmental research on mathematics cognition and mathematics education. Research on infants', children's, and adults' fraction and ratio reasoning allows us to test a range of proposals about the development of numerical cognition that have largely been developed with natural, whole numbers in mind. The developmental research opens several avenues for improving mathematics education. This chapter reviews the empirical research on children's understanding of fraction concepts, symbols, and procedures. It argues for the practical and theoretical importance of examining children's fraction understanding. The chapter describes children's and adults' fraction reasoning, while highlighting the common methodologies in this literature, and presents the developmental picture painted by current research across the life span. It explores the educational implications of developmental research on children's fraction reasoning. Finally, the chapter concludes by considering the directions for future research. [This report is a chapter in: "The Cambridge Handbook of Cognition and Education," edited by J. Dunlosky and K. A. Rawson, Cambridge University Press, 2019, pp. 148-182.]