In this chapter we’ll introduce the Exponential Distribution a one parameter distribution that is a special case of the Gamma distribution and, of course, the Gamma distribution. The Gamma distribution is used to model random durations of time until a next event. What each event is, really only depends on the context of the process being modeled. A general example might be time until the end of the life of someone or something. The Gamma distribution is also used to model random volumes, e.g. rainfall.