Reactive oxygen species (ROS) are a group of free radicals,reactive molecules and ions that are derived from O₂.It is estimated that about 1% of plants-consumed O₂ is converted to ROS in various subcellular compartments such as chloroplast,mitochondria,and peroxisome.ROS are well recognized for playing a dual role as both deleterious and beneficial factors,depending on their concentration in plants.At low concentration ROS acts as second messenger in signaling cascades that mediate several responses in plant cells,whereas at high concentration it triggers oxidative damages to biomolecules and even cell death.Equilibrium between ROS production and scavenging is crucial for plants,and monitored by an efficient antioxidative system consisting of both nonenzymic and enzymic antioxidants.This review thoroughly introduced the current proceedings of studies on plant ROS mainly in terms of their types,production sites,roles in signaling and major damages to plant cells,aiming to provide an insight for improving plant resistance to environmental stress through genetic engineering.