Metal sulfide oxidation is an important process in the past and present global bio-geochemical sulfur cycles. In this process, various sulfur compounds, namely elemental sulfur, polysulfides, thiosulfate, polythionates, sulfite, and sulfate, are generated in different environments. The formation of the sulfur compounds depends on the mineralogy of the metal sulfide and the geochemical conditions in the environment, mainly the pH and the presence of different oxidants. Metal sulfide oxidation can be described by two different pathways: the thiosulfate mechanism and the polysulfide mechanism. Microorganisms play a crucial role in the oxidation of intermediate sulfur compounds, which are formed by the chemical dissolution of the metal sulfides. Under oxic and acidic conditions (e.g., in sulfidic mine waste or in acid sulfate soils), microorganisms oxidize Fe(II) to Fe(III), which serves as an oxidant for the metal sulfides and for most of the intermediate sulfur compounds. Additionally, microorganisms may catalyze the oxidation of intermediate sulfur compounds to sulfate. Under oxic and pH-neutral conditions (e.g., in carbonate-buffered sulfidic mine waste or at the surface of marine sediments) the metal sulfides are chemically oxidized by molecular oxygen via a Fe(II)/Fe(III) shuttle to the metal (hydr)oxide, intermediate sulfur compounds, and sulfate. Microorganisms oxidize the intermediate sulfur compounds to sulfate and, at low partial pressure of molecular oxygen, may catalyze Fe(II) oxidation. Under anoxic and pH-neutral conditions (e.g., in marine sediments), metal sulfides and intermediate sulfur compounds are oxidized either chemically by MnO₂ or by microorganisms using nitrate as an electron acceptor.