Visible injury on vegetation is one of the earliest and most obvious manifestations of ozone injury. However, ozone effects are not limited to visible injury; impacts range from reduced plant growth, decreased yield, changes in crop quality and alterations in susceptibility to abiotic and biotic stresses. Several approaches have been used to estimate the ozone concentrations and exposure durations that induce foliar injury. Most of these studies used short-term exposures (less than 1 day) and measured visible injury as the response variable. In one study designed to estimate the concentrations and durations that would induce specific amounts of visible injury plants were exposed to a range of ozone concentrations and exposure durations and the data were evaluated by regression analysis. The data for several species are summarized to illustrate the range of concentrations required to induce foliar injury on sensitive, intermediate, and less sensitive species. An alternative approach for estimating ozone concentrations and exposure durations which induce foliar injury is the use of limiting value analysis. The limiting values were developed from a review of the literature and represent the lowest concentration and exposure duration reported to cause visible injury on various plant species. The analysis was based on more than 100 studies of agricultural crops and 18 studies of tree species.