The purpose of this paper is to propose a framework for conceptualizing and assessing fidelity of implementation (FOI) in the context of a cluster-randomized design study. A research-validated mathematics curriculum for preschool children called Numbers Plus is used as an example to describe the role of curriculum implementation fidelity in the evaluation of programs, curricula, initiatives, and professional development activities. This three-year project was implemented in two states--Michigan and Indiana. A total of 17 partner programs participated in the project. The authors argue that using various measures of fidelity of implementation provides stronger validity evidence and identifies mechanisms of how implementation works. In addition, the results of the analyses on student and teacher characteristics predicting FOI will be shared. (Contains 1 table.)