The specification, design and implementation of embedded systems demands new approaches which go beyond traditional hardware-based notations such as HDLs. The growing dominance of software in embedded systems design requires a careful look at the latest methods for software specification and analysis. The development of the Unified Modeling Language (UML), and a number of extension proposals in the realtime domain holds promise for the development of new design flows which move beyond static and traditional partitions of hardware and software. However, UML as currently defined lacks several key capabilities. In this paper, we will survey the requirements for system-level design of embedded systems, and give an overview of the extensions required to UML that will be dealt with in more detail in the related papers. In particular, we will discuss how the notions of platform-based design intersect with a UML based development approach.