Interoperability in healthcare is a requirement for effective communication between entities, to ensure timely access to up to-date patient information and medical knowledge, and thus facilitate consistent patient care. An interoperability framework called the Health Service Bus (HSB), based on the Enterprise Service Bus (ESB) middleware software architecture is presented here as a solution to all three levels of interoperability as defined by the HL7 EHR Interoperability Work group in their definitive white paper “Coming to Terms”. A prototype HSB system was implemented based on the Mule Open-Source ESB and is outlined and discussed, followed by a clinically-based example