In this chapter we give a brief look at classical epidemics models. For the reader unfamiliar with these topics, this outline is meant as an introduction to some basic terminology, ideas, and equations that will be needed in Chapter 7 and in other parts of the book. Among the many review papers that give a more thorough picture of this ﬁeld, we mention Hethcote (2000). But of course various chapters are devoted to it in several recent and classical books, for instance, Murray (1989), Anderson and May (1991), Brauer and CastilloChavez (2001), and Edelstein-Keshet (1988). The natural outcome of these studies is the determination of rational vaccination policies to prevent epidemics outbreaks and to control infectious diseases propagation (Dieckmann et al., 2002).