In this chapter we focus on the role of ontologies in developing constraint-based tutors, a special class of Intelligent Tutoring Systems (ITSs). Domain models for ITSs are extremely difficult to develop, and therefore efforts devoted to automatic induction of the necessary knowledge are of critical importance for widening the real-world impact of ITSs. We conducted an initial study which showed that ontologies were useful for manual composition of domain models for constraint-based tutors, as they allow authors to reflect on their understanding of the domain and organize the domain model better. Starting from these encouraging results, we developed ASPIRE, an authoring system for constraint-based tutors, which automated many of the tasks in domain model generation and serves the produced ITSs. The domain ontology plays a central role in the authoring procedure deployed in ASPIRE. We present one of the ITSs produced in ASPIRE as well as the experiences of authors in using ASPIRE.