This paper describes the outcomes of the first strand of the “Applied Research on Science Materials Implementation: Bringing Measurement of Fidelity of Implementation (FOI) to Scale” project. This three-year, National Science Foundation-funded project has two main goals. First, is the measurement of fidelity of implementation of K-8 science and mathematics instructional materials in the Chicago Public Schools (CPS). Second, is the development of a suite of instruments to measure the fidelity of implementation of standards-based science and mathematics instructional materials at the K-8 level. In order to achieve these goals, our first step was the development of a conceptual framework for describing fidelity of implementation. This paper describes the development and components of the FOI Conceptual Framework.