An oxidative burst of reactive oxygen species (ROS) expression is a common response of plants to developmental events and to a number of biotic and abiotic stresses. ROS production physiologically and molecularly regulates development and the stress response and has been proposed as an intracellular second messenger mediating the induction of systematic acquired resistance and the control and regulation of a series of biological processes such as growth, cell cycle, programmed cell death and hormone signaling. The molecular and physiological data indicate functional and mechanistic similarities between the animal and plant NADPH oxidase (NOX), and this enzyme has been considered a major source of ROS production in all life kingdoms. The functions of plasma membrane NOX (PM-NOX) are tightly associated with the production and accumulation of ROS in plants. Here, we report on recent findings in the production and scavaging roles of ROS and the structural features and functions of PM-NOX in plants. The H₂O₂-NOX system may be an important mechanism for developmental regulation and stress response in plants.