Ozone (O-3) is well documented as the air pollutant most damaging to agricultural crops and other plants. Most crops in developed countries are grown in summer when O-3 concentrations are elevated and frequently are sufficiently high to reduce yields. This article examines the difficulties in scientifically determining the reduction in yield that results from the exposure of agricultural crops to surface O-3 and then transforming that knowledge into efficient and effective regulatory standards. The different approaches taken by the United States and Europe in addressing this issue as well as the few studies that have been conducted to date in developing countries are examined and summarized. Extensive research was conducted in the United States during the 1980s but has not been continued. During the 1990s, the European community forged ahead with scientific research and innovative proposals for air-quality standards. These efforts included the development of a ''critical level''or O-3 based on a cumulative exposure above a cutoff concentration below which only an acceptable level of harm is incurred. Current research focuses on estimating O-3 dosage to plants and incorporating this metric into regulatory standards. The US regulatory community can learn from current European scientific research and regulatory strategies, which argue strongly for a separate secondary standard for O-3 to protect vegetation. Increasing impacts Of O-3 on crops are likely in developing countries as they continue to industrialize and their emissions of air pollutants increase. More research is needed on surface O-3 concentrations in developing countries, on their projected increase, and on the sensitivity that crop cultivars used in developing countries have to O-3. The threat of reduced agricultural yields due to increasing O-3 concentrations may encourage developing countries to increase their energy efficiency and to use different energy sources. This could simultaneously achieve a local benefit through improved regional air quality and a global benefit through a reduction in the emission of greenhouse gases.