The article is based on the defense research project “Knowledge Management of the NEC in the Army of the Czech Republic – MENTAL”. The theoretical basis of the project is Topic Maps. The key issue for the project solution is designing and creating a suitable ontology. The implementation environment is technology Tovek Tools and ATOM2. The paper describes the procedure from the selection of an Upper Ontology through the Core Ontology design to the processing of the Domain Ontology. Ontology definitions are stated and their meaning is explained. The paper next explains the ways reusing of an existing taxonomy in an ontology construction, presents the possibilities of using the taxonomy built in the selected domain when creating its domain ontology, explains the difference between taxonomies and ontologies in various contexts, and focuses on the description of the specific domain ontology and the use of the existing taxonomy for its building.