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As plants are confined to the place where they grow, they have to develop a broad range of defence responses to cope with pathogenic infections. The oxidative burst, a rapid, transient, production of huge amounts of reactive oxygen species (ROS), is one of the earliest observable aspects of a plant's defence strategy. First this Review describes the chemistry of ROS (superoxide radical, hydrogen peroxide and hydroxyl radical). Secondly, the role of ROS in defence responses is demonstrated, and some important issues are considered, such as: (1) which of the ROS is a major building element of the oxidative burst; (2) the spatial and temporal regulation of the oxidative burst; and (3) differences in the plant's responses to biotic and abiotic elicitation. Thirdly, the relationships between the oxidative burst and other plant defence responses are indicated. These include: (1) an oxygen consumption, (2) the production of phytoalexins, (3) systemic acquired resistance, (4) immobilization of plant cell wall proteins, (5) changes in membrane permeability and ion fluxes and (6) a putative role in hypersensitive cell death. Wherever possible, the comparisons with models applicable to animal systems are presented. Finally, the question of the origin of ROS in the oxidative burst is considered, and two major hypotheses, (1) the action of NADPH oxidase system analogous to that of animal phagocytes, and (2) the pH-dependent generation of hydrogen peroxide by a cell wall peroxidase, are presented. On the basis of this material, a third 'unifying' hypothesis is presented, where transient changes in the pH of the cell wall compartment are indicated as a core phenomenon in evoking ROS production. Additionally, a germin/oxalate oxidase system which generates H2O2 in response to pathogenic infection is also described.