The oxidative burst is a rapid, transient production of reactive oxygen species (ROS) in plants as an early defense response to pathogen infection. This review discusses the chemistry of ROS (superoxide radical, hydrogen peroxide, and hydroxyl radical), their role in defense responses, and the relationships between the oxidative burst and other plant defense mechanisms. The origin of ROS in the oxidative burst is a major area of research, with two primary hypotheses: the action of NADPH oxidase systems analogous to animal phagocytes and the pH-dependent generation of hydrogen peroxide by cell wall peroxidases. A third hypothesis, a 'uniting' hypothesis, suggests that transient changes in cell wall pH are a core phenomenon in evoking ROS production. The review also explores the impact of the oxidative burst on oxygen consumption, phytoalexin production, systemic acquired resistance, immobilization of cell wall proteins, membrane permeability changes, and hypersensitive cell death. The germin/oxalate oxidase system, which generates H₂O₂ in response to pathogens, is also discussed. Overall, the oxidative burst plays a crucial role in plant defense against pathogens, and understanding its mechanisms is essential for developing effective strategies to protect plants from infection.The oxidative burst is a rapid, transient production of reactive oxygen species (ROS) in plants as an early defense response to pathogen infection. This review discusses the chemistry of ROS (superoxide radical, hydrogen peroxide, and hydroxyl radical), their role in defense responses, and the relationships between the oxidative burst and other plant defense mechanisms. The origin of ROS in the oxidative burst is a major area of research, with two primary hypotheses: the action of NADPH oxidase systems analogous to animal phagocytes and the pH-dependent generation of hydrogen peroxide by cell wall peroxidases. A third hypothesis, a 'uniting' hypothesis, suggests that transient changes in cell wall pH are a core phenomenon in evoking ROS production. The review also explores the impact of the oxidative burst on oxygen consumption, phytoalexin production, systemic acquired resistance, immobilization of cell wall proteins, membrane permeability changes, and hypersensitive cell death. The germin/oxalate oxidase system, which generates H₂O₂ in response to pathogens, is also discussed. Overall, the oxidative burst plays a crucial role in plant defense against pathogens, and understanding its mechanisms is essential for developing effective strategies to protect plants from infection.