The paper discusses the importance of thesauri and ontologies for digital libraries. It deals with standards for metadata representation, namely XML and many related standards. The process of defining a broad-coverage multilingual ontology based on several existing lexical resources is also tackled. A special attention is payed to the incorporation of word-association thesauri that showed to cover many relevant relations essential for the ontology definition. We also present a new software tool that enables to store and efficiently retrieve all the mentioned data.