What makes a productive mathematics discussion? Who does the talking? Whose ideas are taken up? Can a student question take the lesson in an unexpected but valuable direction? What does it mean to be responsive to students' mathematical thinking? These questions intrigued the authors as teachers, teacher educators, and researchers, and they inspired a multiyear project studying mathematics discourse--that cut across teachers, content areas, and grade levels-- to identify key features of productive mathematics discussions. In working with middle-grades teachers, the authors identified a common feature of productive discussions--what they called "responsiveness to students' mathematical thinking." In these interactions, students' ideas were present, attended to, and, at times, used as the basis for instruction. In this article, the authors provide tools to engage students with one another's thinking and to reflect on mathematically responsive classroom interactions.