Understanding what mathematics teachers know, what they need to know about mathematics, and how that knowledge is learned are important goals in mathematics education. Research on mathematics teacher knowledge can be divided into two categories: (a) what knowledge mathematics teachers have or need to have to teach effectively (e.g., Hill, Rowan, & Ball, 2005; Kahan, Cooper, & Bethea, 2003), and (b) how teachers' mathematical knowledge for teaching can be developed (e.g., Bell, Wilson, Higgins, & McCoach, 2010; Proulx, 2008). This book describes research of the second type. To date, research in this area has focused primarily on how mathematical knowledge develops in university or researcher-led teacher preparation or professional development programs. This book is novel in that it concerns how and what teachers learn through the process of teaching itself. In his contribution to this book, Ron Tzur (chapter 3) lays out three reasons why this research is essential. First, he argues, teacher preparation programs simply do not contain enough time for teachers to learn all they need to know, so teachers' learning through teaching is essential. Second, for teachers to develop knowledge of how students think about mathematics and how students receive mathematical lessons, teachers must have classroom experience. Third, the experiences that teachers encounter when teaching have the potential to give rise to meaningful changes in their beliefs and practice.