Effective usage of multimedia digital libraries has to deal with the problem of building efficient content annotation and retrieval tools. MOM (Multimedia Ontology Manager) is a complete system that allows the creation of multimedia ontologies, supports automatic annotation and creation of extended text (and audio) commentaries of video sequences, and permits complex queries by reasoning on the ontology. Categories and Subject Descriptors H.3.7 [Information Storage and Retrieval]: Digital Libraries; H.2.4 [Systems]: Multimedia databases General Terms Algorithms Keywords Video databases, Video annotation, Content-based retrieval, Multimedia ontology, RDF, OWL