One of the most important tasks in the process of designing educational material for distance learning is the representation and modeling of the cognitive domain to which the material refers. However, this representation should be formal, complete and reusable in order to be used by intelligent tutoring system applications, other knowledge domains or tutors. In the context of this work, we propose a methodology that relies on the notion of ontology so as to represent the knowledge domain. Moreover, this methodology has been applied to the educational material of the Hellenic Open University.