This review discusses the production of reactive oxygen species (ROS) by lichens and their possible roles. All organisms produce ROS, and production is increased by many abiotic and biotic stresses. Intracellular ROS production is generally considered to be harmful, and a variety of enzymic and non-enzymic scavenging systems exist to detoxify them. However, extracellular ROS formation has been suggested to play ‘positive roles’, particularly in the response of organisms to stress. Given their high stress tolerance, it is rather surprising that studies on extracellular ROS production by lichens have just started. Surveys of a wide range of lichens have shown that constitutively high rates of extracellular superoxide production occur in the Suborder Peltigerineae, but production appears to be absent in other groups. In some members of the Peltigerineae ROS production is stimulated by desiccation and wounding. It seems probable that the enzymes that produce the superoxide are laccases, based on first the types of substrates that lichens can break down, and second the dependence of the breakdown of these substrates on pH, temperature and the presence of inhibitors. While much more work is needed, we suggest that physiological roles of extracellular ROS production will be found to include defence against pathogens, melanization, and lignin breakdown.