Which is bigger, 1/3 or 1/4? 11/7 or 4/3? Reasoning about the magnitudes of fractions is notoriously difficult for both children and adults. In this talk, Murray will present findings from two studies that utilize different methodologies to explore what makes fractions so challenging. Results from these studies indicate that difficulties have many sources. Some difficulties stem from features of the fractions, such as their symbolic format and their need for units coordination. Other difficulties stem from features of the learners, such as strategy knowledge and attitudes towards mathematics. Finally, time pressure also affects learners’ performance. These factors come together to make fractions challenging for learners.