This paper describes the changes K-8 teachers made in their conceptions of science, teaching, and learning as they participated in a year-long professional development program. The program was based on principles reflected in the national science education reform initiatives, and focused on the role of teachers as decision makers in promoting scientific literacy for all students. The study concludes that: (1) teachers were accepting of examining and even embracing new conceptions of science, but clung to their prior conceptions of science when pressed with uncertainty in a teaching situation; (2) teachers understood the notion of knowledge construction but did not have a clear understanding of the process; and (3) teachers perceived the alignment of science curriculum to national standards to be a time-consuming and sometimes frustrating process. (Contains 34 references.) (WRM)