Reactive oxygen species (ROS) play a key role in plant defense mechanisms. They exert direct antimicrobial action, catalyze the mechanical strengthening of cell walls, function as secondary messengers in the superoxide synthase signal pathway and in triggering the hypersensitive response. Although recent studies have unraveled a nature and the mechanisms of the oxidative burst, many questions related to its mode of regulation, its modulation of signaling networks that control growth, development and defense responses remain unanswered.