This paper presents a framework for examining the Pedagogical Content Knowledge (PCK) required of mathematics teacher "educators" as they endeavour to build the PCK for teaching mathematics that is required of the pre-service teachers with whom they work. The framework builds on existing research into PCK, and provides a series of filters through which to examine the complexity of the work of teacher education. The usefulness of the framework is trialled by using it to study the PCK used by the first author in working with pre-service teachers to build understanding about ways of interacting with students. [This study was supported by a University of Tasmania Teaching Development Grant, and built on the Australian Learning and Teaching Council Priority Projects grant number PP10-1638. Rosemary Callingham, Tracey Muir, and Robyn Reaburn also contributed to the development of the framework.]