A model is an idealized, abstract, and structured representation of real phenomena. Models may manifest as verbal descriptions, physical objects, diagrams, maps, mathematical formulas, and computer programs. Representation of complex dynamic systems increasingly involves computer simulation of alternative model outcomes based on specified input parameters, variables, and relationships. These models link a system's structure to behavior for the purpose of changing a structure to improve behavior. The purpose of a model provides the basis on which its utility must be judged, as all models are by definition simplifications of reality. This entry discusses the modeling process, the assessment of model utility, and the differences among several common modeling paradigms.