Abstract: Belief networks provide an important way to represent and reason about uncertainty - significant factors for modelling students. These networks provide a way of structuring such models, and allow a system to use a systematic approach when gathering information about the scope of the student's knowledge. This work also provides a theoretically-sound way to update the student model, based on the concept of a dynamic belief network. The relationship to related research is discussed. Finally, the paper describes why the barren node concept is important for computational efficiency in belief-net-based student models.