This monograph marks a major step in the growth of the Association of Mathematics Teacher Educators (AMTE) as the leading organization for mathematics teacher educators. It is an effort by AMTE to provide further support for the work of mathematics teacher educators throughout the United States. It is hoped that readers will find this monograph to be useful in their day-to-day work with mathematics teachers. Learning to be a mathematics teacher is a life-long process, but so is learning to be a mathematics teacher educator. Just as mathematics teachers need opportunities for professional growth, so do mathematics teacher educators. Publications such as this monograph can serve an important role in the professional development of mathematics teacher educators.