The notion of pedagogical content knowledge (PCK) was posited in the context of school teaching and the knowledge used by teachers teaching school students. It has been examined for a number of discipline areas, notably mathematics. There are, however, other teaching contexts, including those of teacher educators, whose students are pre-service teachers (PSTs). The content these teacher educators teach is not subject discipline knowledge (or not solely), but the PCK for teaching a subject discipline. What knowledge do teacher educators use as they teach PCK? This paper presents a framework for the PCK required of mathematics teacher educators as they work to develop PSTs' PCK for teaching mathematics. The framework builds on existing research into PCK and categorises aspects of the work of teacher education. The framework's usefulness is examined by studying the PCK used by the first author in building PSTs' understanding of mathematics teacher PCK.