The tutorial will introduce the idea of semantic interoperability and the use of ontologies. Participants will learn what an ontology is and how it can be used. A focus will be on reusing and extending existing ontologies,e.g. SAREF, and getting experience on finding the right ontologies for a given purpose in an ontology catalogue. Finally, ontologies will be used to develop a semantic application for one application domain,e.g. smart home, which will then be extended to semantically interoperate with another application domain, e.g. smart health.