The main goal of the Learning and Teaching Geometry project is to build professional development materials that provide opportunities for teachers to learn about mathematical similarity through the use of videocases, in which specific and increasingly complex mathematical ideas are presented within the dynamics of classroom practice. The central component of the Learning and Teaching Geometry materials is the Foundation Module, which is intended to provide teachers with a thorough grounding in key mathematical and pedagogical issues related to similarity. Field test data indicate that the Foundation Module can promote gains in both teachers' and students' knowledge of similarity. By highlighting the design elements behind this successful professional development effort and documenting the urgent need for further supports in this content domain, the paper has important implications for mathematics educators.