In this article we present an object-oriented approach and a development environment for the system-level design of run-time reconfigurable computer systems. We use the Unified Modelling Language (UML) for the specification, modelling, documentation, and visualization throughout all phases of development, from specification to synthesis. The proposed approach is based on hardware-software co-design and Model Driven Architecture (MDA). This way we allow for thorough and complete system representations, platform-independence, comprehensible and seamless transition from specification to implementation, and the description of common development artifacts and activities. In this article we will focus on aspects and problems which are related to object-orientation, UML, and MDA.