Environmental stresses lead to the production of reactive oxygen species (ROS), which subsequently give rise to hydrogen peroxide (H2O2). At high concentrations, H2O2 can cause oxidative damage to proteins, membrane lipids and other cellular components and leads to programmed cell death (PCD). To protect against these toxic oxygen intermediates, plants possess enzymatic and nonenzymatic antioxidative defense systems. At low concentrations, H2O2 acts as a signal molecule and regulates genes involved in the defense response and other various physiological processes. In this chapter, we describe the biochemistry of H2O2 and its production sites, H2O2 scavenging antioxidant defense machinery, the mechanisms of H2O2 perception and signal transduction, the factors that act downstream and H2O2 interference with other signaling pathways such as calcium and protein phosphorylation networks. These can provide new insights into how plants balance H2O2, as well as describe new H2O2 producing, scavenging and signaling mechanisms.