What is needed to prepare teachers to effectively teach mathematics has been the subject of considerable debate for at least two decades, with the focus shifting back and forth between the number of mathematics courses that teachers need to the mathematics content that teachers need to know. Building upon previous work related to teacher knowledge, some researchers have reconceptualized mathematics content knowledge and have argued that teachers need to know not only the ways that mathematics is used in applied contexts and other professions, e.g., using percentages to compute amounts of discounts, but also the ways required exclusively for teaching, e.g., evaluating the validity of the mathematics in solution methods. Nevertheless, little is known about what mathematics teacher educators, the individuals who are primarily responsible for the mathematical preparation of teachers, should know. Building on their work as part of a two-year professional development project for mathematics teacher educators, and drawing from research on features of high-quality professional development programs, the authors propose a model for the professional development of teacher educators who teach mathematics content courses as part of elementary teacher preparation programs. They illustrate this professional development model by drawing on their ongoing work as part of the Mathematical Knowledge for Teaching Teachers (MKTT) Project, which is a National Science Foundation funded professional development project designed to provide opportunities for mathematics teacher educators to develop their own understandings of mathematical knowledge as it influences their work with preservice teachers in mathematics content courses. While the proposed professional development model has been used only in the context of the project, their aim is to provide insight into the nature of professional development activities designed specifically for teacher educators so that they can develop ways to support teacher educator learning in broader contexts.