The purpose of this evaluation was to examine the impact of i-Ready Personalized Instruction on reading achievement, as measured by SBA scores. We compared students in schools using i-Ready Personalized Instruction school-wide (Treatment students) to students who only received i-Ready Diagnostic assessments and who were in schools only partially using i-Ready Personalized Instruction (Comparison students). The study sample consisted of about 5,632 students in 22 schools from Grades 3-6 from one school district in southern California. All schools used i-Ready Diagnostic assessments, but some schools were considered "full instruction" and assigned all students to receive both the Diagnostic and Personalized Instruction products. Students in these schools were compared to "Diagnostic-only" students in other "partial Instruction" schools where only some students received the Personalized Instruction product in addition to the Diagnostic product. (These Diagnostic and Personalized Instruction students in partial instruction schools are not included in this study.) We did not find that treatment students had significantly higher SBA ELA scores, although directionally, treatment students averaged higher SBA ELA scores than did comparison students. Supplementary analysis of students achieving proficiency in reading on their SBA score showed that treatment did have a statistically significant impact on this outcome. Specifically, students in the treatment group had 1.6 times higher odds of scoring proficient (versus not scoring at least proficient) than their peers in the comparison group. No significant patterns were found in the impact of treatment on specific student subgroups. Overall, this analysis presents promising evidence of i-Ready Personalized Instruction on student reading proficiency. This relationship between i-Ready Personalized Instruction and reading achievement was strongest for students with higher usage of the program. Future studies should seek to further examine the reasons behind this variation in usage.