The main goal of the Learning and Teaching Geometry (LTG) project is to build professional development (PD) materials that provide opportunities for teachers to engage in learning about geometric similarity through the use of video cases, in which specific and increasingly complex mathematical ideas are presented within the dynamics of classroom practice. The central component of the LTG materials is the Foundation Module, which includes 30 hours of PD and 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 supports gains in both teachers' and students' knowledge of similarity. [For the complete proceedings, see ED584443.]