Significance: Accumulating evidence shows that hydrogen sulfide may function as a signaling molecule in processes such as neuromodulation in the brain and smooth muscle relaxation in the vascular system. It also has a cytoprotective effect, since it can protect neurons and cardiac muscle from oxidative stress and ischemiareperfusion injury, respectively. Hydrogen sulfide can also modulate inflammation, insulin release, and angiogenesis. Recent Advances: The regulation of the activity of 3-mercaptopyruvate sulfur transferase (3MST) along with cysteine aminotransferase (CAT), one of the H 2 S producing pathways, has been demonstrated. The production of H 2 S by the pathway, which is regulated by Ca 2+ and facilitated by thioredoxin and dihydrolipoic acid, is also involved in H 2 S signaling as well as cytoprotection. Sulfur hydration of proteins by H 2 S has been proposed to modulate protein functions. H 2 S-sensitive fluorescent probes, which enable us to measure the localization of H 2 S in real time, have been developed. Critical Issues: The basal concentrations of H 2 S have recently been measured and found to be much lower than those initially reported. However, the concentration of H 2 S reached in stimulated cells, as well as the regulation of H 2 S producing enzymes is not well understood. It has been proposed that some of the effects of H 2 S on the regulation of enzymes and receptors might be explained through the properties of sulfane sulfur (S 0 ), another form of active sulfur. Future Directions: The determination of H 2 S concentrations in activated cells using new methods including H 2 S-sensitive fluorescent probes, as well as the investigation of the effects of H 2 S using specific inhibitors, may provide better understanding of the physiological function of this molecule. Clarifying mechanisms of H 2 S activity may also facilitate the development of new therapeutic compounds. Antioxid. Redox Signal. 17, 45-57.