Science testing, as mandated by the No Child Left Behind (NCLB) Act, has refocused attention on the quality of the K-8 science curriculum and instruction in many districts around the country. It has become clear that to improve quality, and meet NCLB requirements, elementary and middle schools must develop different teaching approaches and reconsider some existing notions about the nature of science. Fortunately, as a result of the training and research programs funded by the National Science Foundation (NSF), as well as the work of science educators, reform ideas initiated in the 1990s have been translated into a new generation of science curriculum materials that are yielding positive achievement results in many schools. Future implementation efforts can benefit from the lessons learned in the NSF-funded training projects of the past. However, helping students construct science understanding through a more rigorous curriculum and the use of discovery and inquiry experiences requires teachers to understand far more about science content and processes than at present. The ideas presented in this article are derived from many research and implementation projects as well as more than 11 years of work in the Guiding Education in Math & Science Network (GEMSNET), a partnership of 10 school districts and the University of Rhode Island. Together, they have implemented a comprehensive K-8 kit-based science curriculum and provided ongoing professional development support led by teams of classroom teachers and university scientists and engineers, who volunteer as science mentors. (Contains 3 online resources.)