There is now considerable debate over the kinds of writing tasks that will enable secondary students to learn about science and to demonstrate scientific understanding. Advocates of modernist and postmodernist perspectives propose different genres as necessary to achieve very contrasting agendas. This paper presents an overview and analysis of the claims and counterclaims of these competing positions. Drawing on this overview and research on writing for learning including the authors' classroom-based research, a model is proposed to guide writing for learning in junior secondary science.