Sperm capacitation is an essential process in fertilization. It apparently involves a large number of processes, the common denominator of which is that they donate to sperm the potential to undergo the acrosome reaction, i.e., to release proteolytic enzymes enabling sperm penetration through the egg coat. Although the phenomenon of capacitation has been known for more than 40 years, it is far from understood, and, consequently, there is, as yet, no operational definition of it. The lack of an assay to identify capacitated spermatozoa is both the cause and the effect of this situation. Here we critically review the major changes that are thought to occur during sperm capacitation, and assess their potential use as markers for the identification of capacitated spermatozoa.