To understand how teacher education might build the knowledge it needs, the editors of this issue investigate how organizations outside of education create and maintain "self-improving" systems that enable them to learn how to get better at what they do. To learn about knowledge building in teacher education, the research reported here also looks outside--outside of mathematics teacher education and outside the structure of professional preparation for teaching as it occurs in universities and colleges. This article explores features of a system of collective knowledge building in and for teaching and teacher education that are in place in an international, school-based program for teachers of Italian as a foreign language. We use our study of this unusual teacher education program to investigate how a carefully chosen set of instructional activities, built out of the essential social and intellectual routines of ambitious teaching, can make it possible for novices to teach ambitiously and for teacher educators to build knowledge. (Contains 2 figures and 4 notes.)