The Semantic Web aims to explicate the meaning of Web content by adding semantic annotations that describe the content and function of resources. Providing shareable annotations requires the use of ontologies that describe a common model of a domain. The Web Ontology Language OWL has been defined in order to support representation of ontologies, and their manipulation through the use of reasoning. We provide a brief overview of OWL and the underlying theory, describe applications of ontologies, and give pointers to areas of overlap between Semantic Web and Accessibility research.