This study examines the nature of teacher participation in the context of a video club. Video clubs are professional development meetings in which teachers watch and discuss excerpts of video from their classrooms. In this study, I adopt a situative perspective to examine how teachers develop in their participation to accomplish the goals of the video club. In particular, I examine the roles participants play and explore teachers' participation in four roles that correspond with key goals of the video club. Analysis revealed that teachers' participation shifted in qualitatively different ways over time, with the teachers coming to prompt the group to discuss student mathematical thinking, to propose a variety of interpretations of student ideas, to build on one another's ideas, and to challenge one another's thinking in order to advance the group's conversations. This analysis suggests that the group learned how to participate in roles central to accomplishing the goals of the video club. Studying teacher learning through a lens of participation provides insight into the ways in which teachers coordinate themselves to engage with the goals of professional development and has implications for designing professional development that helps teachers develop practices for teaching mathematics for understanding. (Contains 4 footnotes and 8 tables.)