The teaching of teaching is sophisticated work although it is often viewed simplistically. To challenge simplistic approaches to teacher education, teacher educators need to actively articulate the specialist knowledge, skills and abilities that underpin expertise in teaching and to do so through their practice with their students of teaching. In schools, teachers do not commonly experience a workplace culture whereby the explicit discussion and critique of pedagogical purpose and reasoning occurs. Therefore, it is all the more important that teacher educators bring such thinking to the surface in their teaching about teaching. Teaching is not just about the "doing" of teaching, it is also about the "why" -- which leads to the development of informed and meaningful practice to enhance student learning. This paper considers some of the principles that underpin thinking about teaching as more than transmission and therefore shapes what teacher educators need to know and are able to do.