Metadata are required for finding, exchanging, and managing learning objects. But, their semantics is not clear. Furthermore, the IEEE LOM (Learning Object Metadata) is an IEEE standard since July 2002 but, even if it is widely used, it is not the only one standard used to describe learning objects. The use of ontologies is a key step forward for describing the semantics of information on the Web. We propose to specify the semantics of metadata by using a formal language of ontology, OWL. Furthermore, assuming that we are in an open world and in order to facilitate the exchange of learning resources with other institutions, we propose a solution based on several ontologies.