This chapter examines the interplay between the intended implementation of designed, innovative instructional materials and the enacted curriculum by teachers who are accustomed to customizing their own instruction. When the Education Development Center (EDC) began its studies of implementation fidelity (EDC 2001), it was assumed that teachers would use the materials as the developers intended. The goal of the study was to find out how, in reality, teachers used them. The findings were eye opening but not surprising. Teachers modified, supplemented, and rearranged the materials according to their own beliefs about good teaching and learning and in response to the needs of their students.