The highly diffusible free radical nitric oxide (NO) has emerged as a key signaling molecule in bacteria, plants, and animals. There are several mechanisms through which NO is produced in plants, and once produced, NO readily reacts with various targets, such as thiols and the metallic centers of proteins. During the past few years, S-nitrosylation, the covalent and reversible binding of NO to the thiols of reduced reactive cysteine residues, has emerged as an important posttranslational modification. S-nitrosylation is thought to account for much of the widespread influence of NO on cellular signaling through redox-based biochemical regulation of signaling components. Here, I highlight the emerging roles of S-nitrosylation in plants with particular emphasis on the role of S-nitrosylation in mitochondria during the defense response.