For several decades, sulfur cycling processes in subduction zones have been of major interest to researchers because it is recognized that the sulfur isotope composition and enrichment of the subducting oceanic slab and arc volcanoes are linked. Sulfur, transferred through subduction of the altered oceanic lithosphere, makes a fundamental contribution to the formation of arc-related porphyry ore deposits, magmatic processes, and the redox evolution of the Earth’s mantle due to the transfer of oxidized and reduced sulfur species into the Earth’s interior. In recent years, these topics have attracted new interest, partly due to improvements in computational, experimental, and analytical techniques, including the use of in situ stable isotope measurements in mineral phases or spectroscopic techniques to provide new insights into sulfur speciation under the P–T conditions found in the Earth’s deep interior. In this chapter, we provide a review of current knowledge, and a discussion of sulfur cycling processes in subduction zones. We provide a summary of the diversity of sulfur isotope compositions preserved by oceanic lithosphere formed along fast- and slow-spreading oceanic ridges in the context of inputs into subduction systems. These isotopic variations are largely preserved by exhumed high-pressure rocks. In addition, we show that sulfide minerals in high-pressure rocks can document the subduction and exhumation history of slab material including recording information on the P–T paths, because they can include silicate minerals from prograde to peak to retrograde metamorphic conditions and record variations in fluid compositions, especially sulfur and oxygen fugacities. In particular, the speciation of sulfur in slab fluids has been a major controversy in recent years; both reduced and oxidized sulfur species have been proposed as dominant within subduction zone fluids. These apparent contradictions can be resolved, to some extent, by consideration of slab lithologies and their oxidation state, which pose a major control on the redox state of the released fluids. Hence, in the second part of this chapter we provide an overview of the current discussions regarding sulfur speciation and transport within slab fluids, evidence for sulfur transfer in the subduction zone channel, and finally, sulfur transfer into the overlying mantle wedge, based on the most recent thermodynamic, experimental, petrographic, and geochemical investigations.