This is an introduction to the literature of adoption curves. We focus on two papers: Young (2009) and Brock and Durlauf (2010). First, we discuss how contagion, social influence, and social learning models generate different shapes of adoption curves. Second, we discuss how we may distinguish these models empirically. Third, we consider the Brock and Durlauf model of optimal adoption time/optimal stopping time; and see how this model could generate a few special features in adoption curves.