The purpose of this article was to summarize results from 5 randomized control trials assessing the effects of intervention to improve the fraction performance of 4th-grade students at-risk for difficulty in learning about fractions. We begin by explaining the importance of competence with fractions and why an instructional focus on fractions magnitude understanding may improve learning. Then we describe an intervention that relies strongly on this type of understanding about fractions instruction, and we provide an overview of the intervention's overall effects. This is followed by an overview of 5 intervention components for which we isolated effects. We conclude by discussing some of the lessons learned from this research program.