Free radicals are molecules with one or more unpaired electron(s) commonly found in seminal plasma. Physiologically, free radicals control sperm maturation, capacitation and hyperactivation, the acrosome reaction, and sperm-oocyte fusion. Pathologically, free radicals induce lipid peroxidation, DNA damage and apoptosis of spermatozoa. The present review deals with both the beneficial and detrimental effects of free radicals on sperm function.