The No Child Left Behind Act of 2001 has had significant ramifications across public education. Due to reporting mandates, schools and districts are being held publicly accountable for the academic performance and progress of all students. Since implementation of the law, much attention has been focused on the "achievement gap," that is, any differences in performance between groups of students. Students associated with the achievement gap typically come from certain demographics: minorities, low-income families, students with disabilities, and students with limited English proficiency (English Language Learners). The purpose of this study was to examine the effect of using curriculum-based measures (CBMs) on math achievement, particularly ELL students. Eight elementary schools in northwestern New Mexico, divided into two groups (control and experimental) of four schools each, used the same state-approved, core math curriculum, were compared using a quasi-experimental research design. In addition to the regular core curricular materials, the experimental schools were provided with weekly CBMs, containing sample questions developed from the state's performance standards. Each weekly CBM included at least one question from each of the five broad math strands: number and operations, algebra, geometry, measurement, and data and probability. Fourth (N = 283) and fifth grade (N = 294) students who had continuous enrollment for the duration of the experiment served as subjects. Successive regular administrations of the New Mexico Standards Based Assessment math subtest served as the pre- and posttest measures. Analysis of covariance tests, with the pretest as the covariate, revealed no significant treatment effects for either the fourth or fifth grade students through the use of CBMs as a supplement to the core math curriculum. The significant effects, supported by previous research, were the school and, especially, the teacher for both grades. In this study, the effects of the classroom teacher were of more importance to student achievement than either the school a child attended or what curriculum program or process a given school employed.