In mammals, ejaculated sperm must undergo complicated physiological and functional changes as they traverse the female reproductive tract before they can bind to the zona pellucida, undergo the acrosome reaction, and fertilize the egg. The preparatory changes are collectively referred to as capacitation. Capacitated sperm show hyperactivation, which is a specialized movement of the sperm flagellum that creates the propulsive force for the penetration of the zona pellucida. Changes in sperm associated with capacitation include alterations in surface properties, plasma membrane properties, ionic metabolism, cyclic nucleotide metabolism and protein phosphorylation. Elucidation of the detailed mechanism of capacitation would enable the development of the sperm function assay and the treatment of male infertility.