In plants, reactive oxygen species (ROS) are short-lived molecules produced through various cellular mechanisms in response to biotic and abiotic stimuli. ROS function as second messengers for hormone signaling, development, oxygen deprivation, programmed cell death, and plant-pathogen interactions. Recent research on ROS-mediated responses has produced stimulating findings such as the specific sources of ROS production, molecular elements that work in ROS-mediated signaling and homeostasis, and a ROS-regulated gene network (Neill et al., Curr Opin Plant Biol 5:388-395, 2002a; Apel and Hirt, Annu Rev Plant Biol 55:373-399, 2004; Mittler et al., Trends Plant Sci 9:490-498, 2004; Mori and Schroeder, Plant Physiol 135:702-708, 2004; Kwak et al., Plant Physiol 141:323-329, 2006; Torres et al., Plant Physiol 141:373-378, 2006; Miller et al., Physiol Plant 133:481-489, 2008). In this review, we highlight new discoveries in ROS-mediated abscisic acid (ABA) signaling.