Mammalian sperm are not able to fertilize eggs immediately after ejaculation. They acquire fertilization capacity after residing in the female tract for a finite period of time that varies depending on the species. In 1951, Chang (1951) and Austin (1951) independently demonstrated that such a period of time in the female tract is required for the sperm to acquire their fertilizing capacity. Both authors observed that freshly obtained rabbit sperm introduced into the Fallopian tubes shortly after ovulation were not able to penetrate the eggs; instead if sperm were introduced a few hours before ovulation, the majority of the eggs were later observed to be fertilized. This observation led them to conclude that freshly ejaculated sperm are incapable of penetrating the zona pellucida immediately, and that sperm must remain within the female tract for a period before they are able to penetrate the eggs. Following these original observations, many studies confirmed that the environment of the female tract induces a series of physiological changes in the sperm; these changes are collectively called ‘capacitation’. Inherent to these first observations was that capacitation state became defined using fertilization as end-point.