The teaching and learning of mathematics in K-12 classrooms is changing. New curricula and methods engage learners in working on real problems. An essential feature of this work involves teacher and students in 'talking mathematics'. How can students learn to do this kind of talking? What can they learn from doing it? This book addresses these questions by looking at the processes of formulating problems, interpreting contexts in which problems arise, and arguing about the reasonableness of proposed solutions. The studies in this volume seek to retain the complexity of classroom practice rather than looking at it through a particular academic lens.