A fundamental aspect of simulation is the presence of a model, of a real or imagined system. But how does a simulation model differ from other kinds of models; what is its distinctive feature? Firstly, the models we have considered herein are also, in of and themselves, representations of something else. For example, in a picture, small scale model, or chemical formula, model and representation coincide. A simulation model, conversely, must actually be run if it is to render the phenomenon under examination visible. Specifically, it must produce a process that must evolve autonomously, from an initial to a final state, as if it had its own life.