The abundance of O2 and the highly energetic electron transfer reactions associated with thylakoid membranes make chloroplasts a major source of reactive oxygen intermediates (ROI) in photosynthetic tissues of plants. Attempts to reduce oxidative damage in chloroplasts have included the manipulation of ROI-scavenging enzymes by gene transfer technology. Much of this work has focused on chloroplast-localized superoxide dismutase (SOD), both chloroplast-targeted and cytosolic ascorbate peroxidase (APX) and glutathione reductase (GR). Increased activity of SOD in chloroplasts of transgenic tobacco plants generally leads to increased protection from membrane damage caused by exposure to methyl viologen (MV). In addition, overexpression of chloroplastic Cu/Zn SOD can lead to increased protection from photooxidative damage caused by high light intensity and low temperatures. Transgenic tobacco plants that overexpress APX either in the cytosol or chloroplastic compartments also show reduced damage following either MV exposure or photooxidative treatment and transgenic plants that express increased levels of GR have elevated pools of ascorbate and GSH. Though still preliminary, these results clearly indicate that alterations in the expression of enzymes involved in ROI-scavenging can have significant metabolic effects and provide the hope that this strategy can be used to develop crop plants with increased stress tolerance.