Fractions are fundamental in students’ mathematical development. However, for many students, they are known to be a major stumbling block. In this chapter, we examine the obstacles elementary school children face when they learn fractions, through the lens of numerical cognition. We start by discussing the discrepancy between children’s conceptual and procedural knowledge of fractions, and we review studies showing that the concept of fraction magnitude is particularly difficult to learn. This has wider implications as understanding fraction magnitude has been shown to be a strong predictor of achievement in algebra and overall mathematics achievement in later years. We then discuss the natural number bias (NNB), a well-characterised misconception linked to fraction learning whereby learners are inclined to apply natural number characteristics when reasoning about fractions without considering whether it is appropriate. The NNB is persistent, appearing early in the fraction learning process and lasting through secondary school and beyond. We conclude this chapter by describing interventions aimed at improving fraction learning and we provide suggestions on how to introduce the concept of magnitude more intentionally when teaching fractions.