System-level design presents special simulation modeling challenges. System-level models address the architectural and functional performance of complex systems. Systems are decomposed into a series of interacting sub-systems. Architectures define subsystems, the interconnections between subsystems and contention for shared resources. Functions define the input and output behavior of subsystems. Mission-level studies explore system performance in the context of mission-level scenarios. This paper demonstrates a variety of complex system simulation models ranging from a mission-level, satellite-based air traffic management system to a RISC processor built with MLDesigner, a system-level design tool. All of the case studies demonstrate system-level design techniques using discrete event simulation.