Phytotoxic effects of ozone are described with emphasis on secondary plant metabolism. Numerous ozone-induced genes, enzymes and stress metabolites of antioxidative and phytopathological defense reactions have been discovered for herbaceous plants and forest tree species. Ozone induces reactions normally elicited by viral and microbial pathogens. The molecular basis (receptors, signal chains) for induction by ozone remains to be elucidated. The induced stress reactions seem to change plant predisposition to either enhanced tolerance or susceptibility for a second stressor. The following topics are discussed: ozone and biotic disease, the role of ozone on field sites and ozone limit values.