In order to iteratively specify such adaptive embedded software systems with selfdescribing software components, the Fraunhofer Institute for Cognitive Systems IKS (former Fraunhofer ESK) worked together with the Carnot Institut CEA LIST in France to examine a new development method and tool chain in the project RT-DESCRIBE (Iterative Design Process for Self-Describing Real-Time Embedded Software Components). The software system is initially described in one of the specific prevailing UML models within the respective domain, such as the EAST-ADL architecture description language for automotive systems. This model is enhanced with information for the adaptivity by means of the so-called self-X extension that was developed as part of the project. The domain-independent extension is based on the standardized MARTE UML profile for describing embedded systems. Using the Qompass tool developed by CEA LIST, the designed system model is then enriched with containers encapsulating selfdescription, thus providing a subset of the model information at runtime. With the ERNEST tool developed by researchers at Fraunhofer IKS, the enhanced runtime model can be simulated and analyzed during development, which enables multistage, early validation of the software to be developed. This allows the detection and rectification of errors and flaws in an early stage of development. The results from the simulation environment flow back into the development model so that it can be iteratively optimized. This process reduces the development time and thus the development costs as well.