Much attention has been paid to nitric oxide (NO) research since its discovery as a physiological mediator in the cardiovascular system. In recent years, newer roles have been attributed to this molecule and its close relatives, termed collectively reactive nitrogen species (RNS). These roles relate to different mechanisms of protein modification, among which S-nitrosylation of cysteines has emerged as a potential new paradigm in signal transduction and regulation of protein function. We review here the chemical basis of this modification compared with other protein modifications related to nitric oxide, as well as the kind of specificity we can expect from it. We also review the current methodologies that can be applied to the study of S-nitrosylation and identification of S-nitrosylated proteins in cells, and detail the relevance of this modification in several proteins related to cardiovascular system.