Plant cellular metabolism produces reactive oxygen species (ROS) as a normal product. Excessive production of ROS is caused by various environmental stresses that, in turn, lead to oxidative damage and ultimately cell death. This ROS is used as second messenger for various cellular processes; all of these depend on the equilibrium between ROS production and their scavenging. This chapter describes the generation, sites of production of ROS, and the antioxidative defense mechanism operating in the cells for scavenging of ROS overproduced under various environmental stresses, which requires several enzymatic as well as nonenzymatic components that are discussed in detail.