Research issues have emerged from the rapid introduction of new technologies in government services in order to deliver efficient and cost effective services, information and knowledge through information and communication technologies. However, the complexity of government services and the diversity of actors involved in the processes make the access to the right information difficult and pose several problems. Some problems are linked to the way of presenting and accessing information. Other problems are linked to interoperability among applications and processes of eGovernment services. The objective of the European TerreGov project is to find a solution to such problems. The project focuses on the semantic requirements of governments at local, intermediate and regional levels, needed to build flexible and interoperable tools to support the change towards eGovernment services. We propose, within this project, an ontology to present knowledge and to achieve the required level of semantic interoperability. We use the ontology to describe the domain knowledge of the organization and to index the resources from which civil servants may receive information. The key point of the system is a unique and multimodal ontology used simultaneously for describing domain knowledge, for adding semantics to agency services, for indexing various documents in knowledge bases used by civil servants and finally for supporting the interaction between the users and the system. We present in this paper the challenges of using ontology in eGovernment environments, such as the lack of expressivity of the formalism chosen for interoperability in the project and the risk of inconsistency when the ontology changes. We propose our solution to such challenges and we demonstrate the use of the ontology by the module in charge of managing complex tasks in the system.