Glutathionylation, covalently attaching glutathione(s) to cysteine residue(s) of a protein, has attracted great attention in recent years. The importance of glutathionylation was initially recognized for its role in protecting proteins from irreversible oxidation; however, more studies indicate that glutathionylation is also involved in redox regulation under both normal physiological conditions and oxidative stresses. Potential mechanisms for the formation of glutathionylated proteins have been proposed. Despite the differences among the details of these mechanisms, glutathionylation is generally induced by intermediates including glutathione disulfide, protein-sulfenic acids, and thiyl radical. Taking advantages of proteomics techniques, authors have established methods to identify glutathionylation utilizing 35S-cysteine- or biotin-labeled glutathione, or anti-GSH antibodies. Glutathionylation serves multiple roles in cellular biochemistry, such as modulation of enzymatic activity, glutathione storage, and dynamic regulation of protein function. Development of more efficient methods for glutathionylation identification, systematic investigation of its roles in the context of cellular biochemistry, the interaction with other types of protein modification, and its relevance to some health-threatening diseases will be the wider focus of studies in protein glutathionylation.