Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses that exert adverse effects on plant growth and development. During evolution, plants have evolved complex regulatory mechanisms to adapt to various environmental stressors. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species (ROS), which are subsequently converted to hydrogen peroxide (H(2)O(2)). Even under normal conditions, higher plants produce ROS during metabolic processes. Excess concentrations of ROS result in oxidative damage to or the apoptotic death of cells. Development of an antioxidant defense system in plants protects them against oxidative stress damage. These ROS and, more particularly, H(2)O(2,) play versatile roles in normal plant physiological processes and in resistance to stresses. Recently, H(2)O(2) has been regarded as a signaling molecule and regulator of the expression of some genes in cells. This review describes various aspects of H(2)O(2) function, generation and scavenging, gene regulation and cross-links with other physiological molecules during plant growth, development and resistance responses.