In this paper we consider how mathematics instruction that values, attends to, and builds on students' mathematical ideas is realized through discourse. We describe interactions that build on students' thinking and in which students help to determine the direction of mathematics lessons as responsive. Using a framework we developed to characterize the responsiveness of mathematics interactions, we report the variation in responsiveness across seven middle grades classrooms by describing (a) students' mathematical contributions, (b) the moves teachers enact in response to these contributions, and (c) how these two components interact. We found that there are multiple ways to be responsive to student thinking. [For the complete proceedings, see ED583608.]