Additional information is available at the end of the chapter http://dx.doi.org/10.5772/53360 . IntroductionPhysical exercise, especially moderate physical exercise, can benefit health in a wide range of ways [ -]. Evidence from different age groups, genders and races has revealed that regular physical exercise is associated with high levels of physical fitness and a reduced risk of mortality, while sedentary habits are related to low levels of physical fitness and an increased threat of all-cause mortality [ -]. However, the physiological mechanism of physical exerciseinduced physical fitness remains only partly understood.During physical exercise, the metabolic rate increases greatly, as quantified by oxygen consumption and heat production, which results in an enhanced generation of reactive oxygen species ROS . ROS describe both oxygen-centred free radicals and reactive non-radical derivatives of oxygen resulting from a sequential reduction of oxygen through the addition of electrons. Table shows the representative ROS. The role of ROS in physical exercise is frequently misunderstood. Most people tend to either overemphasize the deleterious role of ROS by maintaining that any ROS generation in vivo during exercise would damage cellular constituents, or underemphasize the beneficial effect of exercise-induced ROS by assuming that the body has enough ROS in vivo that it needs. Two breakthroughs, the identification of ROS in redox regulation and findings on the role of antioxidant supplementation in preventing health-promoting effects of physical exercise, have led scientists to re-examine the role of ROS, especially their positive influence. The goal of this chapter is to outline the current evidence on the sites of ROS generation during exercise, the role of exercise-induced ROS, the effects of antioxidant supplementation on physical-exercise-induced physical fitness, and the mechanism of ROS in exercise-induced adaptation.© 2013 Li; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Oxygen-centred free radicalsReactive non-radical derivatives of oxygen superoxide anion (O 2˙ˉ ) hydroxyl radical (HO˙) peroxyl radical (RO 2˙) alkoxyl radical (RO˙) hydrogen peroxide (H 2 O 2 ) singlet oxygen (1Δg) ozone (O 3 ) Table 1. Name (Radical depiction) of reactive oxygen species (ROS) . Evidence of ros generation in exerciseExercise has been shown to alter oxidative stress in a wide range of body fluids, cells and/or tissues in human beings, rodents and other animals. These include commonly used model tissues, such as blood [ -] and skeletal muscle [ , -], along with many other models less often used for laboratory research, such as neutrophils [ -], lymphocytes [ -], the diaphragm [ , ], liver [ -], heart [ , -], lung [ , ], brain [ -], kidney [ , ], spleen [ , ], and thym...

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