Argumentation is a central goal of science education because it engages students in a complex scientific practice in which they construct and justify knowledge claims. Although there is a growing body of research around argumentation, there has been little focus on developing a learning progression for this practice. We describe a learning progression to understand both students' work in scientific argumentation and the ways in which the instructional environment can support students in that practice. This learning progression describes three dimensions: (1) instructional context, (2) argumentative product, and (3) argumentative process. In this paper, we compare four examples from elementary, middle, and high school science classrooms to explore the ways in which students' arguments vary in complexity across grade level and instructional contexts. Our comparisons suggest that simplifying the instructional context may facilitate students in engaging in other aspects of argumentation in more complex ways. The instructional context may also be used as a tool to support students in argumentation in new content areas and to increase the complexity of their written arguments, which may be weaker than their oral arguments. Furthermore, classroom norms play an important role in supporting students of all ages, including elementary students, in argumentation. © 2010 Wiley Periodicals, Inc. Sci Ed94:765–793, 2010