Most of industry countries are turning their healthcare system towards integrated care paradigms for improving quality, efficiency, and safety of patients' care. Integrated care has to be supported by extended communication and cooperation between the involved healthcare establishments' information systems. The required interoperability level goes beyond technical interoperability and simple data exchange as it has been started in the early world of electronic data exchange (EDI). For realising semantic interoperability, series of standards must be specified, implemented and enforced. The paper classifies standards for health information systems needed for enabling practical semantic interoperability.