Ozone exposure stimulates an oxidative burst in leaves of sensitive plants, resulting in the generation and accumulation of hydrogen peroxide (H2O2) in tobacco and tomato, and superoxide (O2–•) together with H2O2 in Arabidopsis accessions. Accumulation of these reactive oxygen species (ROS) preceded the induction of cell death, and both responses co-occurred spatially in the periveinal regions of the leaves. Re-current ozone exposure of the sensitive tobacco cv. Bel W3 in closed chambers or in the field led to an enlargement of existing lesions by priming the border cells for H2O2 accumulation. Open top chamber experiments with native herbaceous plants in the field showed that Malva sylvestris L. accumulates O2–• at those sites that later exhibit plant cell death. Blocking of ROS accumulation markedly reduced ozone-induced cell death in tomato, Arabidopsis and M. sylvestris. It is concluded that ozone triggers an in planta generation and accumulation of H2O2 and/or O2–• depending on the species, accession and cultivar, and that both these reactive oxygen species are involved in the induction of cell death in sensitive crop and native plants.