The heterogeneity in technology and also in ownership leads to complex systems and interworking problems, which can be seen from the ability to establish and maintain connections with required quality, in fault detection and location, in resource allocation and in charging of the usage of the network’s resources. This is also the reason why researchers look for systems that are adaptive and flexible and provide coverage in various deployment modes (wide area, metropolitan area and local area). Cooperation and coexistence then extends from interworking between different PHY layer modes of the same radio system [1] to cooperation and coexistence at higher layers. One of the main challenges is to consider how and which information to extract from a multitude of parameters, some not directly mapped to each other and belonging to different domains. Another aspect is to ensure that an interworking model is flexible and generic enough to include any new type of network that might be designed in the future and that will require inclusion in the heterogeneous domain. Finally, ubiquitous communication systems and the challenge to deploy them successfully bring also the problem of how to manage the spectrum to ensure successful interworking of the various communication links.