The purpose of this study was to describe how teachers used and adapted a set of curriculum materials that included opportunities for students to engage in scientific practices. Two-fourth-grade teachers in the same school were observed and interviewed. Findings revealed that teachers enacted almost every type of scientific practice in the curriculum, but in ways that varied from the written curriculum materials. Teacher interviews revealed ways in which the teachers understood the rationale for various scientific practices and how to enact them. These findings have implications for curriculum developers, professional development designers, and teacher educators. The study identifies the need to support teachers' understanding of scientific practices and why and how to enact them with their students.