This chapter situates fraction learning within the integrated theory of numerical development. We argue that the understanding of numerical magnitudes for whole numbers as well as for fractions is critical to fraction learning in particular and mathematics achievement more generally. Results from the Delaware Longitudinal Study, which examined domain-general and domain-specific predictors of fraction development between third and sixth grade, are highlighted. The findings support an approach to teaching fractions that centers on a number line. Implications for helping struggling learners are discussed.