Plants are exposed to different types of environmental stressors throughout the different developmental stages. Reactive oxygen species (ROS) are found to play key roles in the maintenance of normal plant growth and improving their ability of stress tolerance. ROS as a secondary messenger performs crucial cellular functions, including the proliferation of cells, apoptosis, and necrosis. Both the external environmental factors and intrinsic genetic programs regulate the morphogenesis of plants. ROS are also considered as by-products of the aerobic metabolism of the plant and are formed in certain cellular compartments like mitochondria, chloroplasts, and peroxisomes. Plants form a huge number of ROS species under unfavorable circumstances that are involved in the regulation of different processes, including programmed cell death, pathogen defense, and stomatal behavior. These reactions often exert irreversible or profound effects on the development of organs and tissues, leading to abnormal death or plant growth. Several molecular approaches to understand the signaling and metabolism of ROS have opened novel avenues in comprehending its key role in abiotic stress. Plants possess their own enzymatic and non-enzymatic antioxidant defense system to encounter ROS. The interconnecting activities of these defensive antioxidants reduce oxidative load and regulate the detoxification of ROS in plants. This book chapter will highlight the importance of ROS metabolism and the role of the antioxidant defense mechanism of plants in combating the deleterious effect of oxidative stress under stressful conditions.