This book is the result of his research around seven years that have been disseminated to several conferences and published in several journals namely Journal on Mathematics Education, Pedagogika, Southeast Asian Mathematics Education Journal, and Journal of Education and Learning. The goal of this activity was a part of the external validation in design research method used in his research and analyze the research result, based on the contributions of the international experts' suggestion in several conferences, especially in design research conference namely South East Asia Design Research. Although the book does not provide a how to do guide for designing and conducting design research, the chapters have been written in such a way that re ect both the conceptual underpinning and practical aspects of the 'what' and 'how' of doing design research. Furthermore, as well as provide the reader an insight into the specifics of doing design research in the domain of learning number operation and learning trajectory of research in mathematics education. He trusts that this chapter will assist the interested reader in getting introduced to this exciting and promising research approach.