The basic reproduction number, R 0 is a measure of the potential for disease spread in a population. Mathematically, R 0 is a threshold for stability of a disease-free equilibrium and is related to the peak and final size of an epidemic. The purpose of these notes is to give a precise definition and algorithm for obtaining R 0 for a general compartmental ordinary differential equation model of disease transmission. Several examples of calculating R 0 are included, and the epidemiological interpretation of this threshold parameter is connected to the local and global stability of a disease-free equilibrium.