This paper describes how elementary teachers' thinking and practice developed as a result of video-based professional development designed to promote reflection on the decision making involved in planning for and implementing mathematics lessons. Of the 189 teachers who attended the video-based sessions, 180 completed post-session questionnaires and a subset were interviewed and observed teaching after the sessions. The video-based sessions were designed to embody many of the characteristics of lesson study, but scaled up to involve a large number of teachers. The majority of teachers believed that their ability to question their students and probe their thinking was extended as a result of the sessions. The teachers who were observed were also able to alter their practice in ways that supported student thinking. The results suggest that these video-based sessions have the potential to impact a large number of teachers across diverse districts. (Contains 1 table.)