A major goal of the H2020 Inter-IoT project is to provide a framework for seamless integration of different Internet of Things (IoT) platforms. To allow the Inter-IoT user to interact with those platforms, which feature their own syntax and Application Programming Interfaces (APIs), so called bridges are designed and implemented. For ensuring interoperability, semantics play a key role. The ontologies used by the different platforms need to be aligned with a central ontology. We present the features supported by Inter-IoT{\textquoteright}s Inter-MiddleWare layer and discuss the operation of a bridge towards a oneM2M based platform.