Reactive oxygen species, highly reactive molecules derived from the metabolism of oxygen, play an important role in the metabolism of spermatozoa, related to the hyperactivated motility and acrosome reaction. Various studies have demonstrated that these molecules, because of their reactivity, can induce peroxidative damage on the cell, especially on the lipid membranes. The spermatozoa, owing to their structure, appear to be very sensitive to the oxidative insult. Various experimental data sustain the hypothesis that, when produced in high amounts, reactive oxygen species can induce sperm damage and become potentially responsible for some forms of unexplained infertility. The cells possess various mechanisms of defense against the oxidative insult, mainly due to antioxidant molecules. An antioxidant treatment can produce an improvement in seminal parameters and sperm function in vitro, and represents a possible form of care for selected forms of infertility.