This chapter begins with a question about the alleged normative character of didactics: To what extent, how and under what conditions can didactics issue value judgements and normative prescriptions about how to organise and manage study processes? Most of the approaches and theories in didactics have an implicit position about this question. The chapter explains the ATD perspective about the object of study of didactics and the corresponding research results. As an example of a law in didactics, it considers the statement about the existence of the didactic contract. This law states that, under certain conditions, a study community formed by students and a teacher, along a study process, behaves according to specific clauses which rule the conduct that students expect from the teacher and vice versa, regarding the knowledge to be taught. According to the ATD, the theory of praxeologies is not only rich enough but also especially useful to describe all the human behaviours relevant for didactics.