Like K-12 mathematics teachers, for whom effective instruction requires a full suite of pedagogical and mathematical knowledge and skills, Mathematics Teacher Educators (MTEs) also need a professional knowledge base to facilitate the development of future educators capable of supporting K-12 students' mathematical learning. In this paper, we theorize about contributors to the development of a Community of Practice (CoP) who were engaged with developing and implementing a common set of instructional materials. We discuss how using the materials in various contexts and interacting with the user community provided a much-needed network of MTEs focused on enhancing their students' mathematical learning experiences as well as their own professional development in terms of teaching, research, and curriculum development. [For the complete proceedings, see ED657822.]