The chapter “Why Do We Model?” addresses the question of why it is important to use models. It is not possible to represent all of reality with a series of mathematical or statistical expressions. Luckily, modelers do not intend to do this. Rather, modelers simplify reality on purpose, so that they can better understand it. However, models must be faithful to reality. In addition, they must be purposeful, that is, they must guide experience in very specific ways. This chapter covers the epistemological basis of ecological models, introduces the core concept of “describe, explain, and interpret” as the core steps of learning from models, and then walks the reader through the process of model development.