Regarding the need of change of both healthcare systems' structure and supporting information systems' architecture to the “Shared Care” paradigm, appropriate health information systems have to be distributed and interoperable, and will usually be based on middleware concepts. Structure and function of information systems meeting such requirements have been analysed in general. To describe, evaluate and combine distributed systems, a generic component model has been used. As a result, CORBA, DHE, and HL7 have been assessed systematically. A way to a common and harmonised middleware approach has been proposed.