The effect of reactive oxygen species (ROS) on spermatozoa can be twofold. Depending on their concentration, moment of activity, and exposure time, ROS can be an essential element in modulating many physiological processes or a cause of serious damage to the gametes. ROS of the greatest importance to the quality of semen are superoxide anion O₂⁻˙ and hydrogen peroxide – H₂O₂. Trace amounts of these compounds are essential for such processes as sperm maturation, capacitation, hyperactivation, acrosome reaction, and the fusion of the sperm with the oocyte. Their excess results in the development of oxidative stress, which significantly reduces the quality of sperm through the peroxidation of lipids, proteins, and other elements of cellular structure. The antioxidative system of semen limits the negative effects of oxidative stress on sperm. The proper function of the reproductive system and successful fertilization depend on maintaining a delicate balance between the amount of ROS and the activity of the antioxidant system.