Raising the level of abstraction is widely seen as the solution for closing the productivity gap in system design. They key for the success of this approach, however, are well-defined abstraction levels and models. In this paper, we present such system level semantics to cover the system design process. We define properties and features of each model. Formalization of the flow enables design automation for synthesis and verification to achieve the required productivity gains. Through customization, the semantics allow creation of specific design methodologies. We applied the concepts to system languages SystemC and SpecC. Using the example of a JPEG encoder, we will demonstrate the feasibility and effectiveness of the approach.