Enabling cloud infrastructures to evolve into a transparent platform raises interoperability issues. Interoperability requires standard data models and communication technologies compatible with the existing Internet infrastructure. To reduce vendor lock-in situations, cloud computing must implement common strategies regarding standards, interoperability and portability. Open standards are of critical importance and need to be embedded into interoperability solutions. Interoperability is determined at the data level as well as the service level. Relevant modelling standards and integration solutions shall be analysed in the context of clouds.