Reactive oxygen species (ROS) or oxidative stress has been reported with strongly involving to pathogenesis of many diseases in human. On the other hand, ROS play a critical regulation as secondary signal to maintain intracellular redox equilibrium. Basically, the antioxidant defense systems in the body counteract with overproduced ROS. However, when the redox balance is broken under severe oxidative stress conditions, it leads to tissue injuries and numerous disorders. In this review, we briefly introduce the systems of ROS and antioxidants systems in the body and discuss the opposite roles of ROS in normal physiological conditions and diseases. For ROS-related diseases, conventional and currently developed antioxidant therapies are also described in this review.