This document reports the work that I developed during the three years of the doctorate school I attended. My research efforts have focused on different aspects apparently disconnected for each other. In practice, during these three years I investigated the Semantic Web research field and this investigation drove me to deal with different topics which are actually parts of the Semantic Web overall project. Hence, my intent has been that of giving a contribution to the overall vision. The main part of this thesis is about ontology interoperability. Furthermore, the use of ontologies for the development of domain-oriented Web sites is investigated and also presented is an ongoing project concerning the development of a semantic domain-oriented search engine. The Semantic Web is the new generation World Wide Web. It extends the Web by giving information a well defined meaning, allowing it to be processed by machines. This vision is going to become reality thanks to a set of technologies which have been specified and maintained by the World Wide Web Consortium (W3C), and more and more research efforts from the industry and the academia. The most important element composing the architecture of the Semantic Web is the ontology. An ontology is the formal definition of a set of semantic concepts, the relations between them, and semantic constraints useful for automatic inference, aimed to describe a specific domain of knowledge. The specification of a Web Ontology Language has been maintained by the W3C. Through the use of ontologies it is possible to deploy information on the Web giving them a machine-understandable form, as well as allowing the automation of tasks, and building domain-oriented Web sites. During the last years the number of organizations which have ``faced’’ themselves on the World Wide Web with their Web sites has incredibly increased and this trend probably will continue in the future. Organizations more and more rely upon these Web portals for offering services to their members or to other people. In order to design, implement and deploy such Web sites fundamentally generic tools are used. I propose to use ontologies as a guide for such tools in order to make easier the task of realizing domain-aware Web sites. In fact, I claim that ontologies can be used for designing Web sites and that the result are Web sites, which have knowledge of the specific domains they support and follow their evolution. Furthermore, this approach allows to easily obtain automatic mechanisms for the semantic annotation of the Web pages composing the Web sites. I present the design of the architecture of the tool WikiFactory, which represents a concrete application of my proposed approach. However, in order to exploit the assets deriving from the use of Web ontologies, they must be specified with the aim of making them interoperable and sharable. In fact, without ontology interoperability it is particularly difficult to exploit the Semantic Web benefits. To this aim the W3C set of Semantic Web technologies provides best practices and mechanisms such as the: the ontology extendibility, ontology patterns, and so on. One issue concerning the ontology interoperability is part of this thesis, and it is also a topic of interest and discussion of the W3C Semantic Web Best Practices and Deployment working group. In particular, the problem that this thesis addresses is that there are two different formalisms that are used for representing meta-information on the Web and both are standards: RDF and Topic Maps. RDF is a W3C standard, it was born with the Semantic Web in mind and represents its base model, while Topic Maps is an ISO standard and albeit it was born for different purposes it is suitable for representing meta-data on the Web. In fact, there is huge amount of interesting and important information represented in Topic Maps that needs to be shared on the Semantic Web as well as that in RDF. The two formalisms are different but present many similarities. This thesis reports the work that I have been doing on this topic. In particular, the study of other existing approaches to solve the problem of the RDF and Topic Maps interoperability and a new proposal concerning a translation model are presented. A tool, named Meta, was developed as well, which supports the defined translation model. Furthermore, much more work has been done. In fact, the tool and the translation model were based on specifications that were non-stable at that time. When new revised and stable specifications were released the translation model and the tool needed to align to them. A new approach for the translation model has been studied and the converter tool has been re-implemented basing on the new approach and specifications.