This mixed instrumental case study aims to conceptualize the notion of engagement in MOOC discussion forums as perceived by students. On the one hand, it builds on the work of Fredricks and his colleagues to explain the potential dimensions of this engagement. On the other hand, the theory of self-determination and its reasons for engagement, the models of motivational dynamics, acceptability and in situ acceptance of technologies were solicited to support the various factors. 127 responses to an anonymous online questionnaire and 11 semi-structured individual interviews, which are being analyzed, highlight preliminary results: engagement in MOOC discussion forums is seen as a quadridimensional construct with thirteen a priori and nine in situ factors.