Although reactive oxygen species (ROS) are highly toxic substances that are produced during aerobic respiration and photosynthesis, many studies have demonstrated that ROS, such as superoxide anion radical (O2(·-)) and hydrogen peroxide (H2O2), are produced in the plant cell wall in a highly regulated manner. These molecules are important signalling messengers playing key roles in controlling a broad range of physiological processes, such as cellular growth and development, as well as adaptation to environmental changes. Given the toxicity of ROS, especially of hydroxyl radical (·OH), the enzymatic ROS production needs to be tightly regulated both spatially and temporally. Respiratory burst oxidase homologues (Rboh) have been identified as ROS-producing NADPH oxidases, which act as key signalling nodes integrating multiple signal transduction pathways in plants. Also other enzyme systems, such as class III peroxidases, amine oxidases, quinone reductases and oxalate oxidases contribute to apoplastic ROS production, some especially in certain plant taxa. Here we discuss the interrelationship among different enzymes producing ROS in the plant cell wall, as well as the physiological roles of the ROS produced.