The purpose of this study is to test the efficacy of a first-grade mathematics intervention called Fusion aimed at developing understanding of whole numbers for students at risk for mathematics learning disabilities. According to results from the National Assessment of Educational Progress, only 42% of fourth graders performed at or above the proficient level in mathematics. Students who perform poorly in mathematics early in school are at risk for continuing to struggle in mathematics throughout elementary school and beyond. There is preliminary evidence that developing in-depth understanding of the whole number system can support learning of future mathematics concepts for students at risk for mathematics learning disabilities. More research is needed on the efficacy of such an approach at a large scale, as well as whether differences in intervention intensity are related to improved student outcomes. This project seeks to fill this gap by studying the efficacy of the Fusion Tier 2 intervention for students at risk for mathematics learning disabilities in classrooms using a response to intervention model.