Companies today routinely utilize computer models to help make decisions. These models take many forms, from simple spreadsheets to sophisticated computer simulations. The chief underlying reason for constructing a model is to assess the impact of a decision on business performance. Based on that assessment, model users will make recommendations and take actions. The tacit assumption is that the model captures the relevant factors at a sufficient level of detail to make accurate projections. The validity of the model thus depends on a host of judgments that the model builder makes in constructing the model—some transparent, others implicit. These judgments are what make building a computer model more of an art than a science.