Because of its facilities for the generalization and specialization of concepts and the unambiguous terminology they provide, ontologies are being used for the representation of reusable software artifacts. This work describes GRAMO, an ontology-based technique for the specification of domain and user models in Multi-Agent Domain Engineering. ONTODUM, an ontology-based tool supporting GRAMO is also introduced. ONTODUM represents the knowledge of GRAMO. Some case studies developed to evaluate GRAMO are also briefly described.