We summarize effects of ground-level ozone (O3) on plants beginning at the atmosphere–leaf interface, and then following with responses at the cellular level and the resulting foliar injury leading to impacts on plant growth. Impacts on crop yield are discussed with the potential for improving O3 tolerance through plant breeding. Impacts of O3 on trees are analyzed with reference to ecosystem functioning and trophic interactions. Facilities to investigate O3 effects on vegetation, O3 phytotoxicity metrics, and O3-protectant chemicals are described.