The two unique benefits of Concurrent and Comparative Discrete Event Simulation are: speed, which is usually 1000 to 10 000 times faster than conventional discrete event simulation; and methodology, which permits the concurrent/comparative simulation of many thousands of experiments. One idea is that a one-for-many experiment, called the reference, is simulated in its entirety, while all others are simulated only where they differ from the reference. A second idea extends the first one; many one-for-many experiments will be significantly more efficient than only one experiment. These two ideas result in tremendous efficiencies, permitting the concurrent simulation of tens of thousands of experiments. The material in the book covers a vast application area in the scientific and business world. For example, in the design experimentation of nuclear power plant operations, many scenarios can be simulated to derive desirable designs or safe operating procedures. Concurrent fault simulation is already a mature technique in the computer aided design of digital systems. Concurrent/Comparative Simulation (CCS) of several instruction sets for a computer can help a designer in making performance tradeoffs. One of the most powerful future applications for CCS/MDCCS (Concurrent and Comparative Simulation/Multi-Domain Concurrent and Comparative Simulation) will be in the testing and debugging of computer programs.