Foliar injury to vegetation is one of the earliest and most obvious manifestations of O3 injury. However, the O3 effects are not limited to visible injury; impacts can range from reduced plant growth, decreased yield, changes in crop quality and alterations in susceptibility to abiotic and biotic stresses. The plant foliage is the primary site of O3 effects, although significant secondary effects can occur throughout the plant. Ozone has been identified as the most important phytotoxic air pollutant in terms of area impacted and as a cause of reduced plant performance. Ozone is an imposed environmental variable that can be detected, monitored and sometimes quantified by observing specific responses on sensitive plants. Ozone impacts a wide range of plant species; these O3-sensitive species and cultivars are distributed throughout the United States. Although O3 has been shown to reduce plant yields, more research is needed to estimate yield loss for additional cultivars and species. More information is needed to determine the characteristics of the exposure that are most important in causing a plant response. Studies need to be conducted to determine the influence of major environmental factors on O3-response thresholds.