This text has started with a minimal representation of traffic on a link, the single-lane deterministic CA with maximum speed one. We have then explored ways to make that model more realistic, for example with respect to fundamental diagrams, or with respect to multi-lane traffic. The focus of this chapter will be to provide some basic underlying theory. Understanding some theory is necessary in particular if one wants to use simple models, because then one needs to understand their deficiencies and the consequences of this.