Recent scholarship has explored mathematical demands faced by mathematics teacher educators and ways to support their development, but little attention has been given to the basic question of how mathematics teacher educators think about content knowledge for teaching. Knowing what they think could inform efforts to support them. Our analysis reveals that some think about mathematical knowledge for teaching as an independent, abstracted resource to be taught and learned in relative isolation from teaching, while others think about it as dynamic, situated work. We argue that this key difference matters for how they work with teachers. Further, our analysis reveals that their thinking about both teaching and justice interacts with their thinking about mathematical knowledge for teaching and that their thinking in these other two domains can be a resource for supporting their mathematical development.