Didactics describes the teaching-learning process which can be conceptualised as a system comprising three subsystems: the teacher, the student and the knowledge at stake, definable as the power of acting. Within this framework, joint action is understood as a unit of the joint attention of teacher and students, joint affordance, and common ground. An analysis of the grammar of didactic action identifies the specific cooperation of the teacher and students. Under this condition, a didactic contract identifies a system of (largely implicit) expectations, and the didactic milieu identifies the structure of the problem at stake, which can be viewed as antagonistic elements producing resistance to the students' action, particularly in some designed cases. Contract and milieu allow us to model didactic activity with learning games which can be seen as a way of modelling the learning process, whereas epistemic games can be seen as a way of modelling the knowledge practice at stake. The examples given show this and how the teacher and students depend on each other. The Joint Action Theory in Didactics (JATD) is descriptive insofar as it allows the identification and description of the logical structure of didactic processes, and at the same time it is normative by allowing the descriptions to be refined. Analysing didactic joint action calls for the cooperation of teachers and researchers, and it needs students as inquirers.