The goal of AMTE Monograph 4, "Cases in Mathematics Teacher Education: Tools for Developing Knowledge Needed for Teaching," is to provide detailed accounts of case use that will inform the mathematics teacher education community on the range of ways in which cases can be used to foster teacher learning and the capacity to reflect on and learn from teaching. The chapters in this monograph describe the use of cases with preservice and practicing teachers at all levels K - 12, in content and methods courses as well as professional development settings, and focus on developing various aspects of teachers' knowledge base (i.e., content, pedagogy, and students as learners). Hence, Monograph 4 should prove to be a superb resource for mathematics teacher educators.