To provide flexibility and efficiency in logic and timing verification of MOS VLSI circuits, it is desirable that various portions of a circuit can be described and simulated at appropriate levels of detail. Such a capability is provided by the Mixed-Mode Simulator described here. This simulator allows different elements of a circuit to be modeled and simulated at different levels of detail. The modeling levels are MOS transistor level, logic gate level and functional level. The simulation levels are timing, multiple delay and unit delay. The simulator is being used on production LSI chips and its performance is discussed.