Drawing on sociocultural learning theory, this book offers a groundbreaking theory of secondary mathematics teacher learning in schools, focusing on the transformation of instruction as a conceptual change project to achieve ambitious and equitable mathematics teaching. Despite decades of research showing the importance of ambitious and equitable teaching, few inroads have been made in most U.S. classrooms, and teacher learning in general remains undertheorized in most educational research. Illustrating their theory through closely documented case studies of secondary mathematics teachers’ learning and instructional practices, authors Horn and Garner explore the key conceptual issues teachers are required to work through in order to more fully realize ambitious and equitable teaching in their classrooms. By theorizing teacher learning from a sociocultural perspective and focusing on instructional practice, the authors make a unique contribution to the field of teacher learning. This book offers researchers, scholars, and teacher educators new theoretical and methodological tools for the elusive phenomenon of teacher learning, and provides instructional leaders and coaches with practical examples of how teachers shift their thinking and practice.