State-of-the-art interoperability between heterogeneous systems on a technical level builds on services oriented architectures (SOA) for a given problem space. Semantics can be introduced by the use of descriptive languages and reasoning algorithms, usually implemented within a message-based enterprise service bus (ESB), the messages triggering service invocation and consumption. Complex event processing (CEP) allows for the definition and detection of situations based on the correlated occurrence of a set of messages, rather than single ones. This paper introduces the technology and lists first references.