There are different strategies to integrate software systems: integration on the data layer, on the business logic layer, and on the user interface layer. The latter area, recently gaining attention by the rising popularity of Web 2.0 mashups, can benefit heavily from formal models and ontologies. In this paper, we argue why integration on the user interface level requires formal semantics. We present a framework using ontologies for user interface integration and discuss the relevant research tasks and links to related research fields. ACM Classification Keywords D.2.2 Software Engineering: Design Tools and Techniques—User Interfaces; D.2.13 Reusable Software: Reuse models General Terms Design, Algorithms Author Keywords Ontologies, User Interfaces, Integration