Oxidative burst is one of the earliest responses in plant resistance to pathogen attack. Studies indicate that the oxidative burst is composed of two phases. The first burst is weak and biologically nonspecific, whereas the phase Ⅱ burst is massive and produced only as an incompatible interaction. The rapid transient production of active oxygen species plays an important role in plant defense strategy against diseases. It involves: 1) antimicrobial activity, 2) substrate for oxidative cross-linking of cell wall, 3) triggering factor of hypersensitive response, 4) mobile signal inducing local and systemic acquired resistance by itself or its derivatives, 5) induction of phytoalexin accumulation, and 6) regulation of gene transcription. Emerging data indicate that the oxidative burst initiates from the activation of NADPH oxidase system resembling that of animal phagocytes. The generation of active oxygen species by a pH-dependent peroxidase is also present in some plants. Further, there is a complete system in plants to regulate the accumulation and scavenging of active oxygen species to protect plants from secondary infection, and at the same time to avoid the oxidative stress.