Global IoT services (GIoTS) are combining locally available IoT resources with cloud-based services. They are targeting worldwide services. GIoTS require interoperability between the locally installed heterogeneous IoT systems. Semantic processing is an important technology to enable data mediation as well as knowledge-based processing. This article explains a system architecture for achieving worldwide semantic interoperability using international standards such as oneM2M and the OMA NGSI-9/10 context interfaces (as used in the European Future Internet Platform FIWARE). Semantics also enables the use of knowledge-based semantic processing agents. Furthermore, we explain how semantic verification enables the testing of such complex systems.