The premise underlying this article is that identifying and using the "five practices model" can make discussions of cognitively challenging tasks more manageable for teachers. By giving teachers a roadmap that they can follow before and during whole-class discussions, these practices have the potential for helping teachers more effectively orchestrate discussions that are responsive to both students and the discipline. The five practices can also help teachers establish a sense of efficacy over their instruction (Smith 1996); they learn that there are ways for them to reliably shape students' discussions to move them toward more powerful mathematical understandings. The authors see these five practices as a focused model for planning a successful whole-class discussion that builds on student thinking while continuing to advance the teacher's mathematical goals for the lesson. (Contains 4 figures.)