Teachers wishing to apply constructivist theories to P-12 pedagogy must skillfully move between student knowledge constructions and powerful disciplinary knowledge and discourses. Although the gulf between these two ways of knowing varies markedly by discipline, constructivist methods are often taught as if they can be applied uniformly across all subjects. This paper provides a critique of overly-simplified applications of constructivism in P-12 pre-service programs by illustrating the way constructivist methods are constrained to differing degrees in the classrooms of three disciplines: literacy, history and mathematics. Building on recent arguments for the need to foreground disciplinary differences in P-12 instruction, the authors draw on published accounts of teaching from the scholarly literature to discuss ways in which pedagogy and discipline interact in praxis. By citing published classroom accounts across three subject areas, the authors illustrate differences in the ways judgment may play out across the disciplines and suggest a framework for future exploration of their method.