The relation between writing in formal schooling and writing in other social practices is a central problem in writing research (e.g., critical pedagogy, writing in nonacademic settings, cognition in variable social contexts). How do macro-level social and political structures (forces) affect micro-level literate actions in classrooms and vice versa? To address these questions, the author synthesizes Yrjö Engeström's systems version of Vygotskian cultural-historical activity theory with Charles Bazerman's theory of genre systems. The author suggests that this synthesis extends Bakhtinian dialogic theory by providing a broader unit of analysis than text-as-discourse, wider levels of analysis than the dyad, and an expanded theory of dialectic. By tracing the intertextual relations among disciplinary and educational genre systems, through the boundary of classroom genre systems, one can construct a model of ways classroom writing is linked to writing in wider social practices and rethink such issues as agency, task representation, and assessment.