System integration in health care ICT includes the necessary mission of standards based interoperability, which includes all layers of information exchange (technical, process, semantics). Open and non proprietary standards play a key role, as they represent the collection of best practices and offer freedom in choosing the most appropriate solution in given situation. After identifying HL7 as the market leader in the health care ICT standardization arena, we have identified several areas of further research, such as formal localization methods and dynamic model definition. The thesis analyzes current status in various countries and markets, studies HL7 domains and protocol properties, references state of the art enterprise architectures, and proposes the additions to HL7 in form of a reference communication model and localization/verification methods. Some of these models have been successfully implemented by the Croatian national eHealth platform. As demonstrated by the results, we strongly believe that this research can bring additional value to the HL7 standard based system implementations worldwide.