System level specification and analysis has become a critical issue in designing today's ever increasingly complex systems. System level design is distinguished from other design activities by its heterogeneous nature. Specifically, systems level designers must integrate information from many different design domains in a single design decision making activity. Rosetta is a systems level specification language that addresses problems specific to systems level specification. Rosetta supports specifying heterogeneous systems by providing mechanisms for: (i) constructing architectures involving heterogeneous components and interconnections; and (ii) specifying multiple views of the same component. Specifications are written in the form of formally defined facets. Each facet extends an associated domain theory, allowing it to be defined using a vocabulary specific to that system facet. Facets are assembled through a facet algebra supporting traditional logical operators plus aggregation. This paper concentrates on system specification using Rosetta.