The aim of the research described in this paper is to construct learner skill profiles. The profiles represent the proficiency in knowledge described by an ontology. For this, we identify three needs: a need for a knowledge reference system that organizes knowledge, for identification of the learner knowledge used in the exercises and for a diagnosis model of knowledge. We describe how we propose to respond to each need. In particular, we propose the calculation of three types of mastery values for the knowledge. The diagnosis model we suggest takes these assessments into account in order to make a profile of each learner.