The purpose of this paper is to contribute to the debate about how to tackle the issue of "the teacher in the teaching/learning process", and to propose a methodology for analysing the teacher's activity in the classroom, based on concepts used in the fields of the didactics of mathematics as well as in cognitive ergonomics. This methodology studies the mathematical activity the teacher organises for students during classroom sessions and the way he manages the relationship between students and mathematical tasks in two approaches: a didactical one [Robert, A., "Recherches en Didactique des Mathematiques" 21(1/2), 2001, 7-56] and a psychological one [Rogalski, J., "Recherches en Didactique des Mathematiques" 23(3), 2003, 343-388]. Articulating the two perspectives permits a twofold analysis of the classroom session dynamics: the "cognitive route" students are engaged in--through teacher's decisions--and the mediation of the teacher for controlling students' involvement in the process of acquiring the mathematical concepts being taught. The authors present an example of this cross-analysis of mathematics teachers' activity, based on the observation of a lesson composed of exercises given to 10th grade students in a French "ordinary" classroom. Each author made an analysis from her viewpoint, the results are confronted and two types of inferences are made: one on potential students' learning and another on the freedom of action the teacher may have to modify his activity. The paper also places this study in the context of previous contributions made by others in the same field.