Modeling is one of the most fundamental processes of the human mind. Yet it is often misunderstood in ways that seriously limit our ability to function coherently and effectively. This Note, reprinted from Artificial Intelligence, Simulation, and Modeling, Widman, Loparo, Nielson (eds.), John Wiley & Sons, Inc., 1989, attempts to define modeling precisely. It surveys the kinds of models human beings use and discusses their advantages and limitations. It places simulation in this context and surveys various kinds of computerized simulation. It then discusses artificial intelligence in the broadest terms, highlighting a few of its most relevant aspects, and attempts to show how artificial intelligence can contribute to — and how it depends on — modeling. Finally, it suggests that the traditional view of simulation is too narrow and should be expanded to encompass more of modeling, leading to "knowledge-based simulation."