The purpose of this paper is to carefully examine the evolution of students' theory building and modeling, critical components of scientific epistemologies, over a unit of study on sinking and floating in one third/fourth grade classroom. The study described in this paper follows in the tradition of Design Experiments ( Brown 1992 , Collins 1990 ) and employs discourse analytic methods in order to examine epistemological development in the dynamic swirl of talk, text and other representations, and social interaction in the classroom. In order to examine students' epistemological development, we draw on the work of Driver et al. (1996) who found that students' use of "theory" falls into three general categories: (1) a descriptive state, or phenomenon-based reasoning, where ideas are not different from experiments, (2) relation-based reasoning, where students come to believe that one can find ultimate proof for particular theories, and (3) model-based reasoning, where students recognize that theories might need to be changed in light of new evidence. Through following students across the unit and examining changes in student participation within discussions and analyzing the quality and content of these discussions, we argue that they are developing a powerful epistemology of science that is more commensurate with the work of actual scientists.