The transfer of a nitric oxide group to cysteine sulfhydryls on proteins, known as S-nitrosylation, is increasingly becoming recognized as a ubiquitous regulatory reaction comparable to phosphorylation. It represents a form of redox modulation in diverse tissues, including the brain. An increasing number of proteins have been found to undergo S-nitrosylation in vivo. These proteins are called S-nitrosothiols, and may play an important role in many processes ranging from signal transduction, DNA repair, host defense, and blood pressure control to ion channel regulation and neurotransmission. This review focuses on the importance of the S-nitrosylation reaction and describes some recently identified S-nitrosothiols in various fields of research.