The "Semantic Web" community poses a new non-functional requirement for web applications> In order to secure interoperability and allow autonomous agent interaction, software for the web will be required to provide machine processable ontologies. We understand that the responsibility, not only for making explicit this requirement, but also to implement the ontology, belongs to requirements engineers. As such, we see the ontology of a web application as a sub-product of the requirements engineering activity. In this tutorial we survey the basic principles behind ontologies as they are being implemented and used by the Semantic Web Community today. Those include ontology languages, tools and construction methods. We focus on a process for ontology construction centered on the concept of application languages. This concept is rooted on a representation scheme called the language extended lexicon (LEL). We demonstrate our approach with examples in which we implement machine processable ontologies in the DAML+OIL language. We finalize with a discussion of today's research issues in ontology engineering, including ontology evolution, integration and validation.