To what extent can computers be used to help teachers create a constructivist learning environment in the science classroom? In this study, data from 23 high school physics classes and 13 teachers were examined to determine the extent to which computers can alter pedagogy and student achievement. Three groups of teachers were examined: Experienced users of the new pedagogy and materials, beginning users of the new pedagogy and materials, and a group of comparison teachers who used traditional instructional methods. Results suggest computers can significantly alter both teaching methods and student achievement; however, there are costs and benefits to the reform. Implications for using computers in the classroom are discussed.  The Constructing Physics Understanding Project (CPU) designed computer-based modular curricular activities, software, and pedagogy to help teachers implement the type of learning environment described in the National Science Education Standards and the Benchmarks for Scientific Literacy (American Association for the Advancement of Science [AAAS], 1993). The purpose of this article is to describe the extent to which teachers integrated these curriculum materials into the classroom to create constructivist learning environments and to investigate the impact on student achievement. In terms of student achievement, results indicate significantly improved students' understanding of physics concepts. When compared to traditional classes, students in CPU classes made significantly higher gains in their understanding of concepts. Relative to national norms, however, the classes of beginning CPU teachers fell somewhere between highly interactive classes and lecture-dominated classes. These results suggest that CPU had a positive impact on students' understanding of physics concepts, but was not able to improve student achievement as much as other interactive approaches.