Reactive oxygen species (ROS) have beneficial or detrimental effects on sperm functions depending on the nature and the concentration of the ROS involved, as well as the moment and the location of exposure. Excessive generation of ROS in semen, mainly by neutrophils but also by abnormal spermatozoa, could be a cause for infertility. Hydrogen peroxide is the primary toxic ROS for human spermatozoa. Low concentrations of this ROS do not affect sperm viability but cause sperm immobilization mostly via depletion of intracellular ATP and the subsequent decrease in the phosphorylation of axonemal proteins. High concentrations of hydrogen peroxide induce lipid peroxidation and result in cell death. On the other hand, the superoxide anion appears to play a major role in the development of hyperactivation and capacitation. The observations that: (i) exogenously generated superoxide anions induce hyperactivation and capacitation; (ii) capacitating spermatozoa themselves produce elevated concentrations of superoxide anion over prolonged periods of time; and (iii) removal of this ROS by superoxide dismutase prevents sperm hyperactivation and capacitation induced by various biological fluids, stress the importance of the superoxide anion in these processes.