The early disease transmission model of Kermack and McKendrick established two main results that are still at the core of most disease transmission models today: the basic reproduction number as a threshold for disease spread in a population;

and the final size of an epidemic. As models become more complex, the relationship between disease spread, final size and $\mathcal{R}_o$ are not as clear. In this talk I will review the notions of the next generation matrix, reproduction number, and final size for the simple epidemic model and discuss their extension to more complex models.