Expertise in mathematics instruction, as commonly recognized, varies from one teacher to another and also affects their teaching performance. Studies on expertise in mathematics instruction are thus important, albeit long overdue, to reveal its specifics. To advance relevant research and practice for the improvement of teacher expertise in mathematics instruction, this book takes a unique approach to present new research from multiple education systems in the East and West. In this introduction chapter, we highlight the background of this book project, three important issues probed in this book, and the book’s content structure and overview.