This review discusses the oxidative burst, an early plant response to pathogen infection. The oxidative burst is a rapid, transient production of reactive oxygen species (ROS), including superoxide radical (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (·OH), which plays a key role in plant defense. The review outlines the chemistry of ROS, their role in defense responses, and the regulation of the oxidative burst. It also explores the relationships between the oxidative burst and other plant defense responses, such as oxygen consumption, phytoalexin production, and systemic acquired resistance (SAR). The review considers the origin of ROS in the oxidative burst, proposing two main hypotheses: the action of a NADPH oxidase system similar to that in animal phagocytes, and the pH-dependent generation of H₂O₂ by a cell wall peroxidase. A third hypothesis is presented, suggesting that transient changes in the pH of the cell wall compartment are a core phenomenon in evoking ROS production. The review also describes a germin/oxalate oxidase system that generates H₂O₂ in response to pathogenic infection. The oxidative burst is generally defined as a rapid production of high levels of ROS in response to external stimuli. The review highlights the differences in plant responses to biotic and abiotic elicitation, and the importance of ROS in triggering defense responses such as hypersensitive cell death. The review also discusses the role of ROS in the induction of phytoalexin production and the regulation of defense responses. The review concludes that the oxidative burst is a key component of plant defense, and that understanding its mechanisms is essential for developing strategies to enhance plant resistance to pathogens.This review discusses the oxidative burst, an early plant response to pathogen infection. The oxidative burst is a rapid, transient production of reactive oxygen species (ROS), including superoxide radical (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (·OH), which plays a key role in plant defense. The review outlines the chemistry of ROS, their role in defense responses, and the regulation of the oxidative burst. It also explores the relationships between the oxidative burst and other plant defense responses, such as oxygen consumption, phytoalexin production, and systemic acquired resistance (SAR). The review considers the origin of ROS in the oxidative burst, proposing two main hypotheses: the action of a NADPH oxidase system similar to that in animal phagocytes, and the pH-dependent generation of H₂O₂ by a cell wall peroxidase. A third hypothesis is presented, suggesting that transient changes in the pH of the cell wall compartment are a core phenomenon in evoking ROS production. The review also describes a germin/oxalate oxidase system that generates H₂O₂ in response to pathogenic infection. The oxidative burst is generally defined as a rapid production of high levels of ROS in response to external stimuli. The review highlights the differences in plant responses to biotic and abiotic elicitation, and the importance of ROS in triggering defense responses such as hypersensitive cell death. The review also discusses the role of ROS in the induction of phytoalexin production and the regulation of defense responses. The review concludes that the oxidative burst is a key component of plant defense, and that understanding its mechanisms is essential for developing strategies to enhance plant resistance to pathogens.