Comprehensive school reform (CSR) is only as effective as its implementation. By using data collected for the National Longitudinal Evaluation of Comprehensive School Reform (NLECSR), this article explores the factors that predict CSR model implementation and the ways that CSR model implementation varies. We found little difference in the fidelity of implementation between schools implementing a CSR program and their matched comparison schools, but we found vast differences in the fidelity of implementation between different CSR program implementation keys (the normative guidelines received from CSR program developers on which we based our implementation measures). The fidelity of implementation is predicted by multiple factors, including the CSR program implementation keys, the principal's instructional leadership, the measure of teachers' professional community, and the usefulness of the CSR developers' assistance. The change in implementation is associated with positive change in principal's instructional leadership and usefulness of the CSR developers' assistance.