An event service is a general purpose facility that provides for observation and notification of events among objects possibly distributed over a network. Despite event-based infrastructures have been available for quite a long time for local-area networks we argue that a number of issues remain to be solved when these technologies are used in a wide-area network setting. In particular, we identify scalability and expressiveness as the two major requirements and we see a number of trade-offs between these two aspects. This presentation is about SIENA, a research project aimed at formulating architectures and algorithms that implement an event service that is scalable as well as expressive. The design of the event service interface will be presented together with a comparison of architectures and routing strategies. Also a validation study based on systematic simulation of network scenarios will be discussed.