Computer simulation is designing a model of an actual or theoretical physical system, executing the model on a digital computer, and analyzing the execution output. Simulation embodies the principle of "learning by doing"-to learn about the system we must first build a model of some sort and then operate the model. Children understand the world around them by simulating (with toys and figurines) most of their interactions with other people, animals and objects. Computer simulation is the electronic equivalent of this type of role playing. It serves to drive synthetic environments and virtual worlds. The paper discusses three primary sub-fields within the overall task of simulation: model design, model execution and model analysis. Graphical output from a physically-based model generated using the program AERO is shown. A manufacturing simulation example is given.