Nitric oxide (NO) has become recognized as a key signaling molecule in plants over the last few years, but still little is known about the way in which NO regulates different events in plants. Analyses of NO-dependent processes in animal systems have demonstrated protein S-nitrosylation - the covalent attachment of NO to the sulfhydryl group of cysteine residues - to be one of the dominant regulation mechanisms for many animal proteins. This reversible protein modification is an important posttranslational, redox-based regulation mechanism for many proteins of different classes in animals. For plants, however, the importance of protein S-nitrosylation remained to be elucidated. This chapter will discuss the chemistry of S-nitrosothiol formation and the release of NO from S-nitrosylated cysteine residues, as well as the specificity and regulation of S-nitrosylation. Furthermore, the identification of plant proteins as candidates for this type of protein modification, and the physiological functions of protein S-nitrosylation in plants are described.