We analyse the construction as well as the role of ontologies in knowledge sharing and reuse for complex industrial applications . In this article , the practical use of ontologies in large-scale applications not restricted to knowledge-based systems is demonstrated , for the domain of engineering systems modelling , simulation and design . A general and formal ontology , called P HYS S YS , for dynamic physical systems is presented and its structuring principles are discussed . We show how the P HYS S YS ontology provides the foundation for the conceptual database schema of a library of reusable engineering model components , covering a variety of disciplines such as mechatronics and thermodynamics , and we describe a full-scale numerical simulation experiment on this basis pertaining to an existing large hospital heating installation . From the application scenario , several general guidelines and experiences emerge . It is possible to identify various iewpoints that are seen as natural within a large domain : broad and stable conceptual distinctions that give rise to a categorization of concepts and properties . This provides a first mechanism to break up ontologies into smaller pieces with strong internal coherence but relatively loose coupling , thus reducing ontological commitments . Secondly , we show how general and abstract ontological super theories , for example mereology , topology , graph theory and systems theory , can be used and reused as generic building blocks in ontology construction . We believe that this is an important element in knowledge sharing across domains . Thirdly , we introduce ontology projections as a flexible means to connect dif ferent base ontologies . Ontology projections can occur in simple forms such as include-and-extent and include-and-specialize , but are in their richest form very knowledge-intensive , being in fact themselves full-blown ontological theories .