Dynamics studies time varying systems, such as the climate, population growth, planetary motion, and chemical reactions. Dynamicists formulate mathematical models of the systems, analyze the models, and predict the behavior of the systems. They seek simple models whose predictions agree with many observations. Once they develop an accurate model of a system, they can augment experimentation and observation with mathematical analysis and computer simulation, thus predicting (and perhaps controlling) its behavior more quickly and more easily. For example, fluid flow models reduce the need for wind tunnel tests of new aircraft, saving the designers time and money. An accurate model of the climate could predict the effects of alternate industrial policies on global warming without the cost, risk, and lead time of experimentation.