This paper describes 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 is working measure the fidelity of implementation of standards-based science and mathematics instructional materials at the K-8 level in a large urban school district through the development and application of a suite of FOI instruments. This first strand focused on the development of a conceptual framework for describing FOI. We began with a thorough review of previous work on FOI, then engaged in an iterative process of theoretical review and framework development. We determined that the framework needed to comprise categories of critical components (the essential elements of the intended program model) and engaged in a process of science instructional materials review that focused on identifying of these critical components and placing them into framework categories. This paper will describe the application of the framework to the development of instruments, and the ways this framework will help to accurately describe teacher use of science instructional materials and support our ability to provide useful, meaningful information about FOI to the school district being served.