For decades the National Science Foundation has been funding the development of instructional materials whose design is based upon the recommendations of educational research. These recommendations include the idea that learning be sequenced and organized using an experiential learning cycle or an instructional model such as the Biological Science Curriculum Study (BSCS) 5E Instructional Model (Engage, Explore, Explain, Elaborate, and Evaluate). More recent research studies such as those synthesized in "How People Learn" suggest that instruction address students' prior knowledge, help students connect new knowledge to a rich framework of big ideas, and support students in monitoring and taking control of their own learning. Over time, the work of curriculum developers in response to these recommendations has resulted in an extensive portfolio of research-based instructional materials that span the sciences disciplines. However, few researchers have systematically explored how these materials influence student learning and the role that implementation fidelity plays in the materials' ultimate impact. This article addresses this issue by exploring data collected in two BSCS research studies. The authors briefly describe these studies, their respective findings, and make recommendations for teacher professional development. (Contains 2 figures and 4 tables.)