Reactive oxygen species (ROS) are produced in plant tissues exposed to a variety of biotic and abiotic stresses. Much attention has focussed on superoxide and hydrogen peroxide as the major ROS produced in plants, particularly with respect to their roles in signalling. In contrast, singlet oxygen, a highly reactive form of molecular oxygen, has received relatively little attention. However, recent reports indicate that it too has a specific role in signalling in plants. We have used a novel bioluminescence imaging technique to demonstrate that wounded leaves emit small but significant quantities of light. Furthermore, this low-level endogenous bioluminescence is attributable to the generation of singlet oxygen. Our current results suggest that wound-induced singlet oxygen generation is a consequence of disruption of chloroplast electron transport in cells immediately surrounding the site of damage.