The publish-subscribe paradigm has been a received considerable attention over recent years. Many models for topics have been explored, ranging from regular expressions to hierarchical addressing. We believe that the expressiveness of the topic is a crucial factor in determining the utility of publish-subscribe systems, and we thus adopt an ontology-based system. Ontologies promise is "a shared and common understanding of a domain that can be communicated between people and application systems" (6). The usage of ontologies in the publish-subscribe system allows publishers to generate events within a specific domain, and allows subscribers to submit subcriptions that can match a whole or partial event. Prior knowledge obtained from previous events can also be used in conjunction with a current event when evaluating subsriber queries. This means that a single event can have a different semantic meaning across different subscriptions and prior knowledge within the ontology plays a role in the evaluation process.