Application integration can be performed on three levels: the data source level, the business logic level, and the user interface level. Approaches of the latter kind include, e. g., portal and mashup solutions. Deeper integration offering rich interaction possibilities usually leads to strong dependencies between applications, and requires deep knowledge of those applications. To reduce such dependencies, ontologies have been proposed as an inter-lingua mediating between the applications. While there have been approaches for using ontologies for integration on the data source and business logic level, this dissertation is the first approach for ontologybased application integration on the user interface level. It discusses the development of suitable formal ontologies, the use of reasoning and rule-based event processing, as well as aspects such as performance and seamless cross-technological interaction.