Epidemiologic studies on male infertility suggest that many cases should be considered idiopathic infertility, that is an exact cause of the inability to induce conception cannot be identified. Recently it was found that the redox status within the male gamete or in the semen can be at least partly responsible for the etiology of infertility. Each living cell is capable of producing reactive oxygen species (ROS), such as superoxide anion, hydroxyl radical, singlet oxygen, or hydrogen peroxide. This process involves the male germ cells as well. A certain amount of free radicals generated in the respiratory chain is necessary for the normal function of sperm cells. In cases of overproduction of ROS, the antioxidant potential of sperm cells can be exhausted and oxidative stress may develop. Prolonged exposure of sperm cells to ROS may cause peroxidation of the cell membrane lipids, alter the structure of protein receptors, enzymes, and transporter proteins, and affect sperm DNA fragmentation. This review aims to summarize the current state of knowledge regarding the redox system in male sperm and the consequences of oxidative stress on semen quality and sperm cell function.