In this paper, I argue that mathematics teachers’ professional knowledge matters, and so requires specific attention in mathematics teacher education. Two examples from studies of mathematics classrooms in South Africa are described, and used to illustrate what mathematics teachers use, or need to use, and how they use mathematics in their practice: in other words, the substance of their mathematical work. Similarities and differences across these examples, in turn, illuminate mathematics teachers’ professional knowledge, enabling a return to, and critical reflection on, mathematics teacher education.