I will develop and then reflect on two inter-related claims in this chapter. The first is that the sets of concepts that have emerged through research on mathematics knowledge for teaching (MKT), while relatively recent, have nevertheless proliferated. This is not surprising given that as part of educational knowledge, it is part of a horizontal knowledge structure with a relatively weak grammar (Bernstein, Br J Sociol Educ 20(2):157–173, 1999). The second is that a key ‘new’ position producing and produced by this knowledge development is that of mathematics-teacher-educator-researcher working simultaneously as knowledge producer and recontextualiser in the university. A number of questions, about research and practice emerge from the grammar of MKT and the dual, perhaps ambiguous positioning of its agents. This chapter thus offers a story about mathematical knowledge for teaching framed by Steve Lerman’s contributions to the field, and the possibilities evoked for further work.