This introduction presents an overview of the key concepts discussed in the subsequent chapters of the book. This book aims to privilege the context of the digital age by offering a comprehensive text that highlights the impacts this has, and will continue to have, on the aspects of learning and teaching mathematics in formal education systems and settings. In the intervening years, the mathematics education research field has expanded to consider the teacher in all of his/her roles within the context of both pre-service and in-service teacher education, with increasing emphasis on how to prepare and support teachers to conduct their professional lives in this fast-moving digital age. The book adopts a historical approach to the analysis of major national or regional professional development initiatives in Colombia, Italy, Mexico and Portugal to aim at making explicit the “pragmatic theories” that have underpinned their design, implementation and (in some cases) evaluation.