This paper examines fundamental assumptions about the notion of transforming subject matter, which is widely regarded as a core practice of teacher work, a crucial feature of teacher knowledge and a measure of teacher expertise. First, the notion of transforming subject matter and the ways it has been taken up in Anglo-American discourses of teacher knowledge are discussed in relative detail. Second, the paper examines and questions fundamental, yet mostly unexplored, assumptions, including the individual teacher as the locus of transformation, the possessor of the content knowledge at stake and the gatekeeper who enables students to access subject matter content. Finally, these widespread assumptions are problematised against the background of French and German traditions of didactics. These traditions do not regard the capacity to transform subject matter as a characteristic of an individual teacher, but rather of social and cultural systems that are institutionally contextualised and oriented towards normative conceptions of education.