Successful implementation of evidence-based educational practices at scale is of great importance but has presented significant challenges. In this article, the authors address the following questions: How does the level of on-site technical assistance affect student outcomes? Do teachers' fidelity of treatment implementation and their perceptions of school climate mediate effects on student performance? Using a randomized control trial at scale, the authors examine Kindergarten Peer Assisted Learning Strategies, which previously has been shown to be effective in increasing student reading achievement. Analyzing data from 2 years and three sites, the analyses show that the level of on-site technical support has significant effects on reading achievement gains, are robust across multiple sites, and are mediated by fidelity of implementation within teachers' classrooms. (Contains 2 figures and 4 tables.)