This paper reviews research on the effects of ozone on stomatal behavior, and draws attention to the direct link between stomatal function and air pollution. Contradictory results concerning the effects of air pollutants on stomatal activity are attributed to the use of univariate statistical treatments of data. Alternative experimental methods that subject stomata to a functional test are described. These methods verify the hypothesis that ozone predisposes trees to drought stress. It is proposed that ozone directly attacks the walls of guard and subsidiary cells, leading to impairment of the sensory mechanism of the stomata.