Few elementary school teachers command a science background sufficient to offer children early and sustained opportunities to develop an understanding of science concepts, and facility with scientific modes of reasoning. This four-year, NSF-funded study was designed to investigate how and to what extent particular combinations of science content and pedagogical learning experiences for teachers in grades 4-5 produce an impact on teachers' knowledge for teaching and on their students' learning. In this paper, the authors report results from the first phase of this work, which addressed the fundamental questions of whether the professional development courses did, in fact, lead to teacher and student knowledge gains, and what the effects were for the three course models that were tested, in relation to the control group and to one another. (Contains 10 tables.)