This chapter aims at clarifying the purposes of the development of Comparative didactics in the French-speaking educational research, and the potential of this research field for addressing curriculum issues from a bottom-up perspective i.e., from classroom studies. First, certain salient characteristics of the development of subject didactics in French-speaking research are reminded to explain the conditions of the emergence of comparative didactics as a comprehensive science of knowledge transmission in teaching and learning practices. The notion of “didactic system” and “didactic transposition” are central to the conceptualization of the generic features of teaching and learning. Epistemological and methodological issues inherent to the analysis of dynamic systems are examined to highlight the necessity of defining a “tertium comparationis” for comparing (or relating) teaching and learning practices about different knowledge contents, in different cultural contexts, and at different time scales. The Joint action framework in didactics (JAD) is presented as a response to this need, functioning as a generic framework anchored in the analysis of the content specificity of teaching and learning practices. This theoretical development allows didactic research to shift from the subject structure of knowledge contents to the contents built in classroom transactions as the departure point of the analysis. This shift opens new perspectives for understanding the consequence of curriculum reforms from the perspective of the teachers and the students.