The study of the development and use of digital resources in mathematics education involves a growing number of theoretical constructs, some widely used in mathematics education, others developed to meet the specific needs of this area of research. This characteristic is not peculiar to educational research on digital resources but is broadly shared by mathematical education research at large. It motivated the enterprise now known as the networking of theories in which researchers of diverse origins and cultures have been collaborating for nearly two decades. In this chapter, we use the perspectives and methodological tools offered by networking research to reflect on the role of theories in studying the development and use of digital resources in mathematics education. After highlighting the diversity of theories and theoretical combinations mobilized in this area of research, we introduce the idea of networking of theories and some associated tools. We then focus on constructions and studies sharing a common denominator: the reference to the instrumental approach, either through direct reference to Pierre Rabardel and Pierre Vérillon’s seminal ideas and publications (Rabardel, 1995; Vérillon & Rabardel, 1995), or through reference to the transpositions made of these ideas in mathematics education. We especially focus on three examples that illustrate the diversity of theoretical combinations in play and their affordances: the European project ReMath, the extension of the instrumental approach to the teacher and the documentational approach to didactics.