This chapter examines univariate (single variable) probability distributions that are used frequently to model random phenomena. Discrete probability distributions are introduced first, followed by continuous probability distributions. Discrete distributions can be used to model the number of failures until a successful rocket launch, the number of passing students in a class, or the number of taxis that pass a street corner, as well as many other phenomena with countable outcomes. Continuous distributions are used to model measurement variables such as weight, height, and time. Joint distributions will be introduced in Chapter 5.