Reactive oxygen species are highly reactive intermediates that are produced during the biochemical processes of oxygen with other .organic molecules. Although the generation of ROS are known to have physiologic functions in cells, if levels of ROS production become high enough to overwhelm the antioxidants in these cells, these molecules causes lipid, DNA, and protein damage. This is called as oxidative stress which is considered to be an important factor in the etiology of many pathologic conditions. Understanding the effects of ROS on sperm functions is important to improve new diagnosis and treatment strategies for male factor infertility.