The production, knowledge and learning of mathematics follow a certain combination of "logics": a logic of knowledge, a logic of the subject and a logic of situations. They do not coincide. How do they relate to didactical activity?Do the epistemological, semiological and psychological approaches suffice as theories of didactical activity? What is the importance of the latter with respect to the first two? Is it necessarily reduced to the description of practices, or to a collection of techniques or technologies?Knowledge is the most important means and object of the transmission of acquisitions from one generation to another, and also the most powerful means of influencing nature and people. Its production and transmission have become one of the principal activities of all of humanity. Studies of the conditions for creation and transmission of knowledge now have high priority.