This study investigates mathematics teacher learning in a video-based professional development environment called "video clubs." In particular, the authors explore whether teachers develop professional vision, the ability to notice and interpret significant features of classroom interactions, as they participate in a video club. Analysis for the study is based on data from two year-long video clubs in which teachers met monthly to watch and discuss video excerpts from each others' classrooms. Participating in a video club was found to influence the teachers' professional vision as exhibited in the video club meetings, in interviews outside of the video club meetings, and in the teachers' instructional practices. These results suggest that professional vision is a productive lens for investigating teacher learning via video. In addition, this article illustrates that video clubs have the potential to support teacher learning in ways that extend beyond the boundaries of the video club meetings themselves. (Contains 11 tables and 8 notes.)