ContentCVS: A CVS-based Collaborative ONTology ENgineering Tool. We present ContentCVS, a system that implements a novel approach to facilitate the collaborative development of ontologies. Our approach adapts Concurrent Versioning, a successful paradigm in collaborative software development, to allow several developers to make changes concurrently to an ontology. Conflict detection and resolution are based on novel techniques that take into account the structure and semantics of the ontology versions to be reconciled by using precisely-defined notions of structural and semantic differences between ontologies and by extending existing ontology debugging and repair techniques.