This review summarizes the current knowledge on oxidative stress of plants from the viewpoint of formation and degradation of reactive oxygen species (ROS) in plant tissue. Special attention was paid to various non-enzymatic and enzymatic ROS-producing systems, including sites of ROS production (chloroplast, mitochondria, peroxisome, endoplasmic reticulum, plasma membrane, apoplastic space and cell wall). Defence reactions of plants, employing various antioxidants and antioxidant enzymes are also described in detail.