This article examines teachers' perspectives on the challenges of using a science reform curriculum, as well as their learning in interaction with the curriculum and parallel professional development program. As case studies, I selected 4 veteran teachers of 2nd or 3rd grade, with varying science backgrounds (including 2 with essentially none). Capitalizing on the first activity of the monthly teacher meetings, wherein each teacher described an issue/challenge in using the curriculum, I analyzed how their conceptualization of issues changed over 5 points in time, 2 in the fall of 1 school year and 3 in the spring of the next. Exit interviews extend the time span under analysis to 32 months. Across this span, I examined the extent to which their thinking about the issues they raised reflected the problematic, in Dewey's sense of catalyst for fruitful taking up of the problem in ways supporting learning. Given the importance of understanding science as a way of knowing, I also analyzed their thinking vis-a-vis S. Carey and C. Smith's (1993) scientific knowledge-construction continuum from "knowledge unproblematic" to "knowledge problematic". Analyses indicate that 3 of the 4 achieved fundamental advancements in their thinking, although of different forms and through different mechanisms of change. (Contains 3 footnotes.)