The design of complex systems requires an enormous amount of modeling and simulation at the various levels of detail to assure high quality results. Most design methodologies require the use of separate modeling languages and simulation environments for each refinement step and simulation technique. Since one of the major goals of design is to reduce the time to market for a given system, the current approaches are becoming unacceptable. A new unified modeling (UM) methodology is under development at the University of Virginia which provides a top-down/bottom-up design methodology based on a single modeling environment. This single environment methodology is made possible through a technique known as hybrid modeling, which enable the simulation and analysis of behavioral components within a system-level model. By simulating behavioral components along with components at the system modeling level, high risk portions of the design can be examined at earlier stages in a given product's development; thus reducing the time to market. The paper examines the hybrid modeling techniques which make this multi-level modeling possible.