Reasoning and sense making are at the heart of mathematics teaching and learning throughout the K-12 mathematics curriculum. Focusing on these skills gives students the opportunity to analyze, interpret, and critically think about the mathematics they learn. The terms "reasoning" and "sense making" are frequently used to describe the wide range of mathematical processes that relate to one's ability to think about and use mathematics meaningfully. Mathematical reasoning frequently denotes one's ability to draw conclusions on the basis of assumptions and definitions. Mathematical sense making refers to one's ability to critically analyze concepts, situations, and contexts and draw connections among them to other knowledge. How can teachers help their students develop the reasoning and sense-making skills necessary to see mathematics as a sense-making discipline? One opportunity comes from actively engaging students in problem solving, encouraging them to listen and question one another (including the teacher), and justify their mathematical work. This article explores a professional development program in which middle school teachers solve problems, assign the same problems in their own classrooms, and analyze their students' thinking. (Contains 4 figures.)