Sulfur isotope measurements have been conducted since the late 1940s and have revealed interpretable isotopic variability in a wide variety of natural systems. Applications of sulfur isotope measurements have included studies of the geologic history of microbial life on Earth, conditions of the early Earth and solar system, processes affecting the interior of the Earth, and the processes occurring on the Martian surface, just to name a handful. Interpretations of sulfur isotopic variability in natural systems are underpinned by the known or characterizable isotope fractionations associated with (bio)geochemical processes such as chemical and microbially mediated reactions. In this chapter, we review the existing constraints on sulfur isotope effects and fractionations associated with inorganic sulfur species in aqueous and gaseous systems. These are relevant to a variety of freshwater, marine, and sedimentary environments, as well as in hydrothermal and volcanic systems, microbial metabolism, and extraterrestrial environments such as Mars. This chapter focuses on summarizing constraints involving all four stable sulfur isotopes (32S, 33S, 34S, and 36S). We also provide new constraints on isotopic “clumping” in S–O bonds (34S–18O) of inorganic aqueous and gaseous species for isotopically equilibrated systems to highlight the potential applications of clumped isotopes to the study of sulfur in natural systems.