This article explores the potential of innovative, reform-based, middle school mathematics curricula to impact the professional development of teachers. Specifically, this 4-year study (N = 131 participants) examined the degree to which repeated experiences with reform-based curriculum activities impacted prospective teachers' mathematical content knowledge, their thinking about how children learn mathematics, and the formation of their philosophical perspectives and pedagogical strategies. Findings and assertions were drawn from multiple data sources (interviews, discussions, classroom observations, student work, etc.) collected throughout the duration of the project. The findings suggest meaningful shifts in the perspectives of these teachers with respect to mathematics teaching and learning, and show indications of growth in their own content and pedagogical content knowledge.