Science can be understood as a vast repository of models. Given a natural system, one may select the models that are most appropriate for whichever purpose one has in mind. Eventually, one can tweak the model as needed. This conception lies at the heart of the Newtonian way of doing science, and it underpins our understanding of science and modeling as representational activities. A representation here is a kind of encoding – that is, a map – between a natural system and a model. This chapter explores the basic features of the modeling relation, the interplay between systems as referents and systems as models.