"Mathematics teacher educator-researcher-mentor" is a mouthful, but an accurate title to describe the range of roles of mathematics teacher educators across the nation and around the world. There is limited research about the professional development of teacher educators and even less research specific to mathematics teacher educators. This article draws from the existing studies about mathematics teacher educator development and additional mathematics education literature to examine Shulman's (1986) notion of curricular knowledge for mathematics teacher educators and how growth in curricular knowledge can be facilitated. The analysis builds upon a previous investigation, a self-study that examined the knowledge growth and structure of a novice mathematics teacher educator in terms of all three of Shulman's (1986) categories of teacher content knowledge: (1) subject matter content knowledge; (2) pedagogical content knowledge; and (3) curricular knowledge. All three categories merit more in-depth attention for considering the work and professional development of mathematics teacher educators. The purpose of this report is to elaborate upon the notion of curricular knowledge. The author first summarizes Shulman's (1986) notion of curricular knowledge and illustrates how the ideas play out for mathematics teacher educators. She then summarizes the existing research about mathematics teacher educator professional development and discusses how this research furthers a conceptualization of curricular knowledge. The author concludes with insights related to further understanding the professional development of mathematics teacher educators. (Contains 3 figures.)