Curriculum Associates' "i-Ready® Instruction" is a supplemental, online personalized instruction program available for reading and mathematics. The Human Resources Research Organization (HumRRO), in collaboration with Century Analytics, implemented a quasi-experimental design (QED) using 2018-19 "i-Ready Diagnostic and Instruction" data to evaluate the impact of Curriculum Associates' mathematics "i-Ready Instruction" on student mathematics achievement at grades K-5. It was hypothesized student achievement, as measured by the "i-Ready® Diagnostic," would be higher for students using "i-Ready Instruction" for mathematics over a comparison group of students who did not use this instruction. Matching was conducted to identify a set of comparison students demographically similar to the "i-Ready Instruction" treatment students for each grade level. First, the sample was stratified by gender, English learner status, disability status, and economic disadvantage status. Next, propensity score matching was used to identify analytic samples of "i-Ready Instruction" and comparison students matched on baseline mathematics student achievement. Students who received the "i-Ready Instruction" and students in the comparison group were administered the mathematics "i-Ready Diagnostic" assessments. To evaluate impact, hierarchical-linear modeling (HLM) was conducted separately for each analytic sample with students at level 1 and school at level 2. Results suggest students using "i-Ready Instruction" with fidelity performed statistically significantly better on mathematics performance than students in grades K-5 who did not use this instruction. The effect sizes fall at the upper end or exceed (in the case of kindergarten) the range for which recent research by Kraft (2019) has found is typical of education interventions. These findings provide support that, when used with fidelity, student use of "i-Ready Instruction" for mathematics is tied to higher student mathematics achievement.