A question that confronts teachers of mathematics is how best to establish in their classrooms a space where students can participate more fully in the curriculum. This article explores the development of agency in the school mathematics classroom. Using data from a variety of classroom contexts, this article employs sociocultural constructs of theorists such as Lev Vygotsky and James Wertsch to examine teacher and student activity during classroom interactions. A specific focus of this examination is the development of student agency. In particular, I explore practices that recognize the tension between the agency of the people teaching and learning the mathematics and the agency of the discipline that accredits and conventionalizes ways of knowing and doing the mathematics. The article concludes that it is in the teacher-student negotiation of coming to know and do mathematics that the development of student agency can be actualized.