Tropospheric ozone (O₃) may adversely affect tree growth, with critical levels for O₃ being exceeded in many parts of Europe. However, unequivocal evidence for O₃-induced foliar injury on woody species under field conditions has only been found in a few places. Visible O₃ injury appears to occur mainly in the Mediterranean Basin, which is also the area where the least amount of information is available on O₃ exposure as well as the sensitivity of individual species. Overall, the quantitative risk assessment of O₃ impacts on mature trees and forests is vague at the European scale, as most knowledge is derived from controlled O₃ fumigations of young trees, grown in isolation in exposure chambers. Research suggests that risks exist, but these need to be validated for stand conditions. O₃-induced changes in resource allocation rather than productivity appear to be crucial as they affect competitiveness and predisposition to parasite attack and may eventually lead to the loss of genetic diversity. 'Free-air' O₃ fumigations in forest canopies may reveal processes that are susceptible to O₃ stress under field conditions and provide a scientific basis towards quantitative risk assessment and realistic definitions of critical levels for O₃ in forest ecosystems.