Although there are increasing numbers of professional development (PD) materials intended to foster teachers' mathematical knowledge for teaching within the topics of number and algebra, little attention has been given to geometry. In this article we describe the Learning and Teaching Geometry project's approach to the development of PD materials aimed at supporting middle grades teachers to meet the challenges of teaching similarity. We discuss the design research approach of the project, including developing a mathematical learning trajectory, videotaping in classrooms, and selecting videoclips to support our learning goals. We use a sample videocase to illustrate the design process. (Contains 3 notes and 4 figures.)