This open access book is the product of an international study which offers a state-of-the-art summary of mathematics teacher collaboration with respect to theory, research, practice, and policy. The authors – leading researchers and teachers on mathematics teacher collaboration – represent a wide range of countries and cultures. Chapters explore the various forms of teacher collaboration; the diversity of settings and groupings in which mathematics teacher collaboration occurs; the tools and resources that support mathematics teacher collaboration and are the product of collaboration; and the breadth of outcomes of such collaboration. Teachers’ experiences and learning in collaborative settings are represented through their own voices as well as the voices of researchers. Forms and outcomes of collaboration are considered through a variety of theoretical perspectives and methodological approaches. The authors reflect on the policy implications of this work and suggest new directions of research that take into account contextual, cultural, national and political dimensions that impact teachers’ work and learning through collaboration. The book is a valuable resource for researchers, practitioners, and policy makers who are interested in the power of teacher collaboration, and its history and potential for promoting educational innovations and equitable experiences for all teachers and learners.