The purpose of this evaluation was to examine the impact of i-Ready Personalized Instruction that met Curriculum Associates' recommended usage levels on mathematics achievement, as measured by the Massachusetts Comprehensive Assessment System (MCAS) mathematics assessment. This study compared mathematics achievement growth of students who used i-Ready Personalized Instruction with fidelity to comparison to that of students who only used the i-Ready Diagnostic Assessment suite, but did not have access to Personalized Instruction. Hierarchical linear modeling was used to compare mathematics achievement gains, and analyses were performed by grade level. Results showed that students in Grades 3, 5, 6, and 8 that used i-Ready Personalized Instruction with fidelity had significantly higher MCAS mathematics achievement gains in the 2020-21 school year than did comparison students who did not use i-Ready Personalized Instruction. These results help to illustrate the importance of students meeting Curriculum Associates' recommended usage levels of the Personalized Instruction suite in order to maximize the potential benefits of this instructional program.