This chapter overviews sulfur chemical speciation and behavior in different types of hydrothermal fluids across the lithosphere, spanning from active geothermal systems and basinal brines to metamorphic and magmatic–hydrothermal fluids. The information on sulfur in these fluids stems from a wide range of recent analytical, experimental, and computational methods whose advantages and limitations are discussed. A special emphasis is given to in situ approaches that have enabled unprecedented insights into the fascinating sulfur chemistry in hydrothermal fluids. These insights motivated a critical review of the chemical speciation of key base, precious, and critical metals whose behavior is intimately linked to sulfur. The results allow the role played by the different sulfur ligands on hydrothermal transport of metals and ore deposit formation to be evaluated. An outline of major challenges and emerging perspectives for sulfur-related research in geological fluids ends up this chapter.