The effects of ozone on Mediterranen forests. Tropospheric ozone (O 3) is the most important air pollu tant because of its effects on forests and relationships with climate change. Results obtained by IPP-CNR in recent years suggest Mediterranean forests are less O 3 sensitive than they were thought before. This does not imply that Mediterranean forests are free of any O 3 damage. Visible foliar injury, stomatal alterations (increa sed night-time transpiration, delayed closure in response to water and light variations), increased susceptibi lity to other stressors (drought, weak parasites) as well as background O 3 levels exceeding European stan dards, confirm O 3 pollution as a serious concern to Italian forests. To protect forests from O 3 effects, Europe uses one only exposure index, i.e. AOT40, for the entire continent. Researchers now know that O 3 exposure does not summarize the real potential of O 3 injury to vegetation. A new concept of "flux" has been develo ping and includes avoidance through stomatal closure and tolerance through detoxification and damage re pair. Validating the O 3 flux approach implies a great deal of scientific effort. Meanwhile, different AOT40 th resholds on a geographic or plant-type basis may be tested.