This chapter discusses the nitrosation of proteins. Nitrosation reactions introduce the nitric oxide (NO) group into an organic molecule. When covalently bound to a carbon, nitrogen, oxygen, or sulfur atom, the incorporated NO is referred to as a “nitroso group”; when coordinated to a transition metal, it is referred to as a “nitrosyl group.” With the identification of NO as a product of many cell types and because of the well-known reactivity of this radical and its activated redox forms, the possibility that nitrosation may occur under physiological conditions has been entertained. Considerable experimental evidence is mounting that protein nitrosation occurs with NO generation in vivo. Nitrosation of proteins provides a means to store NO and represents a form of posttranslational modification that modulates protein function and cell phenotype.