Incorporating algorithm and architecture level design space exploration in the early phases of the design process can have a dramatic impact on the area, speed, and power consumption of the resulting systems. This paper proposes a framework for supporting system-level design space exploration and discusses the three fundamental issues involved in effectively supporting such an early design space exploration: definition of an adequate level of abstraction; definition of good fidelity system-level metrics; and definition of mechanisms for automating the exploration process. The first issue, the definition of an adequate level of abstraction is then addressed in detail. Specifically, an algorithm-level model, an architecture-level model, and a set of operations on these models, are proposed, aiming at efficiently supporting an early, aggressive system-level design space exploration. A discussion on work in progress in the other two topics, metrics and automation, concludes the paper.