\({\text{CO}}_{2}^{- }\) , Cl−, and F-bearing aqueous fluids can play an important role during local to regional metamorphism ranging anywhere from low to high grades. Sulfur in these fluids, which can be present over a range of oxidation states from H₂S to \({\text{SO}}_{4}^{2 - }\) , can significantly influence the oxygen fugacity of the rock during metamorphism. Additionally, sulfur can form complexes with a wide range of major and trace elements, thereby enhancing their mobility or promoting the precipitation of sulfides and other minerals. By itself, the presence of sulfur has important implications with regard to various silicate-oxide-sulfide mineral reactions and the relative stability of these minerals in P-T-X-fS2-fO2 space. This has important implications with respect to the stability and persistence of regional sulfide-bearing ore deposits in metamorphic terranes and the mobility of various trace elements such as Au, Th, U, and REE. In this chapter, our current understanding regarding sulfur’s presence and role in mass transport, sulfide ore formation, sulfide versus oxide mineral stabilization and growth, and the formation and stability of sulfur-bearing scapolite during metamorphism is explored for a variety of metamorphic terranes. These terranes range from regionally metamorphosed terranes to metamorphism of sulfide-bearing ore deposits to high-grade metamorphism in the mid to lower crust (amphibolite- and granulite-facies), and, lastly, to high-pressure, low-temperature metamorphism during blue schist- and eclogite-facies metamorphism, which also includes subduction-related processes.