Given its high level of phytotoxicity and distribution of elevated concentrations over broad geographic areas, O3 is considered the most critical air pollutant affecting vegetation in the United States. Diverse experimental methods have been used to assess the impacts of O3 on the crop yield. Comparisons of plant growth and yield in charcoal-filtered or unfiltered air and the use of chemical protectants show that ambient O3 levels will reduce the growth and yield of numerous plant species. Ozone studies in open-too field-exposure chambers have provided exposure-response functions needed to evaluate the economic impacts of O3 on agriculture. Exposure-response functions have been developed for a range of legume, grain, fiber and horticultural crops.