The production of reactive oxygen species (ROS) by the consumption of molecular oxygen during host-pathogen interactions is termed the oxidative burst. The most important ROS are singlet oxygen (¹O₂), the hydroxyperoxyl radical (HO₂·), the superoxide anion, hydrogen peroxide (H₂O₂), the hydroxyl radical (OH ^-) and the closely related reactive nitrogen species, nitric oxide (NO). These ROS are highly reactive, and therefore toxic, and participate in several important processes related to defence and infection. Furthermore, ROS also play important roles in plant biology both as toxic by-products of aerobic metabolism and as key regulators of growth, development and defence pathways. In this review, we will assess the different roles of ROS in host-pathogen interactions with special emphasis on fungal and Oomycete pathogens.