Understanding the interaction between internally constructed and externally imposed aspects of the teaching context may be the missing link between calls for school reform and teachers' interpretation and implementation of that reform. Although the context of the local school culture has a profound impact on teachers, there are other external forces that are specifically aimed at influencing teachers' pedagogical and curricular decisions. These externally imposed aspects of context include some of the existing tools of reform, such as national standards, mandated state core curricula, and related criterion-referenced testing. However, little is known about how these reform tools impact teachers' thinking about science and science teaching or how teachers respond to such tools. This study examined the interactions between individual teachers' beliefs about teaching and learning science in elementary school and the tools of reform that are imposed upon them. Comparative case studies were conducted in which two elementary teachers' science instruction, teaching context, and related beliefs were examined, described, and analyzed. In this study, the teachers' fundamental beliefs about science and what it means to teach and learn science influenced their interpretations of the sometimes contradictory messages of reform as they are represented in the standards, mandated curriculum, and end-of-level tests. Suggestions about what these findings mean for needed aspects of teacher professional development are offered.