In recent years, the gaseous molecule nitric oxide (NO) has been shown to be involved in many important biological events. S-Nitrosothiols are biological substances derived endogenously from NO, and are found in a variety of tissues exhibiting NO-mimetic activity. Fundamental studies on the chemical aspects of S-nitrosothiols have become an integral part of NO research, with a view to further understanding the numerous biological functions both of NO and of S-nitrosothiols themselves. It has often been suggested that Snitrosothiols represent a means either for the storage or transport of NO, although the evidence for this is sparse. Many S-nitrosothiols have now been synthesized chemically, and at present they show the most promise as clinically useful NO-donor drugs. In this review, we examine in detail the biological functions of Snitrosothiols, as well as the chemical properties and biomedical applications of these compounds.