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Fault simulation of circuits described at multiple levels of abstraction (RT, gate, switch) is a major problem in the area of CAD and testing. Although the concurrent paradigm is generally acknowledged as the most efficient, several techniques are crucial to successfully extend it to multilevel simulation of large circuits. In particular, based on multilist traversal, fraternal event processing, list events, and levelizing, advances are presented here in simulation speed, accuracy, and generality. For zero-delay elements, the simulation of irrelevant activity is avoided, but the accuracy of structural (interconnect) logic simulation is maintained. What is described here has been implemented in MOZART, and detailed experimental results are reported. Relative to the good machine, the average faulty machine is simulated 900 to 17 000 times faster. The approach presented is not restricted to fault simulation, and is thus applicable to the new area of concurrent case simulation.