Oxygen can occur in different chemical configurations, which can be more active and even toxic for both microorganisms and higher life forms. It has been shown in man that these active forms in fact play a dual role. They can cause serious diseases but their toxicity can also protect man, and indeed plants, against attacks from pathogenic microorganisms. This paper recounts the history and leads on to describe the most recent work to demonstrate that the active forms of oxygen(including H2O2 and the superoxide ion) are involved in plant defence machanisms against fungi, bacteria and viruses. H2O2 is directly involved early on in the hypersensitivity reaction that characterizes an incompatible pathogen/host interaction: the pathogen is neutralized very rapidly causing only a few mini-necroses. It also probably involved at a later stage in establishing a wide range of defence mechanisms that necessitate the derepression of host genes.