In certain real-world problems, we have data indicating that two given variables are related to each other, but we might not actually know a particular equation that can predict the value of one variable from the other. In such a case, our goal is to find a formula that provides a very good model of the relationship between the variables. Such a formula is often the first step toward using other mathematical techniques to analyze the real-world situation. In this chapter we consider the problem of finding a function or formula that fits a given data set as closely as possible.