Cell signaling is a complex and highly regulated process. Post-translational modifications of proteins serve to sense and transduce cellular signals in a precisely coordinated manner. It is increasingly recognized that protein S-nitrosylation, the addition of a nitric oxide group to cysteine thiols, serves an important role in a wide range of signaling pathways. In spite of the large number of SNO-proteins now identified (∼1000), the observed specificity of S-nitrosylation in terms of target proteins and specific cysteines within modified proteins is incompletely understood. Here we review the progress made in S-nitrosylation detection methods that have facilitated the study of the SNO-proteome under physiological and pathophysiological conditions, and some factors important in determining the SNO-proteome. Classification schemes for emergent denitrosylases and prospective 'protein S-nitrosylases' are provided.