The development of complex object behaviour in domains of interactive graphical applications such as desktop-Virtual Reality (desktop-VR) or multimedia is still a problem today. We attribute this situation to the lack of sophisticated concepts and tools for preimplementation behaviour specification and provide the tool-supported visual modelling language SSIML/Behaviour to address this problem. As an extension of the Unified Modeling Language (UML) our language allows the platform-independent behaviour description and can be easily integrated into UML tools. Moreover, it is possible to map SSIML/Behaviour models to platform-specific program code automatically.