The Semantic Web is a “web of data” whose goal is to enable machines to understand the semantics, or meaning, of information on the World Wide Web. The Web Ontology Language OWL and its successor OWL 2 are “object-oriented” languages for defining and instantiating Web ontologies. Ontology is a term borrowed from philosophy that refers to the science of describing the kinds of entities in the world and how they are related. The choice of which profile to use in practice will depend on the structure of the ontologies and the reasoning tasks at hand. Strictly speaking, SPARQL is a query language for data that is stored natively as RDF or viewed as RDF via middleware. From a logical point of view, its logical counterpart is the well-known notion of conjunctive/disjunctive query. The chapter also presents an overview on the key concepts discussed in this book.