This paper presents the results of a mixed methods project that examined 12 districts’ implementation of K–5 mathematics materials. The authors found that fourth grade mathematics achievement gain scores were significantly correlated with the teacher’s perception of principal support for implementing mathematics instructional materials, whether teachers had a designated weekly grade-level planning time, and whether it was part of the designated mathematics support person’s role to support the implementation of the mathematics instructional materials. The authors also found that while percent low income and study year cohort predicted 23.5% of the variance in mathematics gains, schools with teachers who perceive their principals to be more supportive of the implementation of the instructional materials contributed an additional 3.9% variance in gains in mathematics achievement. Results should be of interest to school administrators and policymakers in efforts to advance mathematics program improvement—shedding light on factors affecting the use of such materials at large scale. The results may be particularly useful as districts consider strategies to address the Common Core State Standards for Mathematics and determine the role instructional materials will play in those efforts.