Brassinosteroids (BRs) play important roles in the complex network of plant signal transduction that regulates plant growth and development. Field and greenhouse trials have shown that exogenous BRs can also improve plant tolerance to abiotic and biotic stress. We have recently shown that application of exogenous BR enhances while inhibition of endogenous BR biosynthesis compromises the tolerance to photo-oxidative and cold stresses and resistance to cucumber mosaic virus in cucumber plants. These results suggest a possible role of endogenous BRs in plant stress responses as well. We have further shown that BR-induced stress tolerance is associated with increased accumulation of reactive oxygen species (ROS), which, in turn, is important for BR-induced stress tolerance. BR-induced ROS accumulation is sensitive to inhibitor of plasma membrane-bound NADPH oxidases. ROS mediate BR-induced stress tolerance, most likely by regulating genes involved in plant stress response pathways. Given their established roles as second messengers, ROS may also participate in other BR-regulated biological processes including plant growth, development and photosynthesis.