This article examines the pedagogical tensions involved in trying to usestudents' ideas as the basis for class discussion while also ensuring thatdiscussion is productive mathematically. The data for this study of theteaching of one middle-school teacher come from observations andvideotapes of instruction across a school year as well as interviews with theparticipating teacher. Specifically, the article describes the teacher'sattempts to support a student-centered process of mathematicaldiscourse and, at the same time, facilitate discussions of significantmathematical content. This tension in teaching was not easily resolved;throughout the school year the teacher shifted his emphasis betweenmaintaining the process and the content of the classroom discourse.Nevertheless, at times, the teacher balanced these competing goals by usinga ``filtering approach'' to classroom discourse. First multiple ideas aresolicited from students to facilitate the process of student-centeredmathematical discourse. Students are encouraged to elaborate theirthinking, and to compare and evaluate their ideas with those that havealready been suggested. Then, to bring the content to the fore, the teacherfilters the ideas, focusing students' attention on a subset of themathematical ideas that have been raised. Finally, the teacher encouragesstudent-centered discourse about these ideas, thus maintaining a balancebetween process and content.