The No Child Left Behind Act resulted in increased school-level implementation of assessment-based school interventions that aim to improve student performance. Diagnostic assessments are included among these interventions, designed to help teachers use evidence about student performance to modify and differentiate instruction and improve student outcomes. The mCLASS: Math software (Ginsburg, Cannon, Eisenband, & Pappas, 2006) is comprised of screening/progress monitoring curriculum-based measures (CBMs) and Diagnostic Interviews to help teachers identify students' skill levels. mCLASS: Math enables teachers to target instruction to each student's needs and monitor each student's progress toward mastery. Educators are expected to use the constantly updated diagnostic information to improve ongoing instruction and increase student achievement. Schools have found that the use of mCLASS: Math helps support curriculum, instruction, and assessment (Ginsburg et al, 2006). This study provides evidence about the effectiveness of mCLASS: Math in improving student outcomes on a statewide math achievement test, examining whether or not using the mCLASS: Math CBM impacts students' achievement test scores. Among the students who were administered the CBM, we investigate whether or not being administered the mCLASS: Math Diagnostic Interviews further influences subsequent test scores. This study was conducted in a large mid-western state which provided data from 175 districts, 606 schools, and 1856 teachers for this study. All the schools and teachers had elected to implement mCLASS: Math; however, there are instances in which individuals students may be exempted from the assessment. CBM measures quickly identify students who are at risk for poor mathematical performance while Diagnostic Interviews identify students' mathematical strengths and weaknesses which can be used to inform instruction. Overall, the administration of CBM measures provides broad, useful information about students' skill levels and identifies those students in need of further assessments and development. Tables are appended.