The study of air pollution effects on vegetation has made rapid progress in the last five years. Growing concerns about effects of future increases in temperature and carbon dioxide (C0 ) levels on plant life have altered 2 the perspective of plant biologists in the field of pollutant-plant inter actions. In many cases, it is anticipated that crops and trees will increasingly experience multiple stresses in an altered environment: an environment in which physiological processes will no longer be matched to climate. Because of this problem, a major part of the focus of the air pollution effects research has shifted since 1987. Moreover, recent advances in our understanding of plant metabolic and molecular responses to stress have made it clear that many abiotic stresses elicit similar fundamental mechanisms. Adaptation responses to drought, extremes of temperature, xenobiotics and air pollutants are now known to involve the response of both specific and common resistance mechanisms, which often include altered gene expression. The field of air pollution effects on vegetation has benefitted greatly from this unification since results obtained and advances made in allied fields are now directly relevant. The advent of molecular genetics has made possible the production of transgenic plants containing altered amounts of resistance gene products which enables the posing of experimental questions which could not be addressed only five years ago. Hypotheses concerning the relevance of specific metabolites and processes to known responses to air pollution stress can now be tested."