Functions play an important role not just in mathematics, but in many fields of science. They describe how variables are related to one another and enable a deeper understanding of that relationship. In economics, we are particularly interested in real functions, i.e., functions that describe relationships among real numbers. As important as it is to model the phenomenon, it is as equally important to understand the effect that change in one economic variable has on the others. In this chapter we introduce the basic terms related to functions.