The crystal structures and chemical compositions of sulfide minerals are summarized briefly before going on to review their redox chemistries, particularly the roles played by bacteria. In the formation of sulfide minerals, two processes need to be considered; one applicable to all sulfide systems is the microbial reduction of sulfate, the other is microbial reduction of metals, especially iron. Sulfate-reducing prokaryotes (SRP) can supply reactive sulfide ions for the formation of sulfide minerals. The SRP, of which there are more than 120 species, are ubiquitous in many anaerobic environments, although marine sediments are the most important. The SRP are able to grow under extreme conditions of pH and temperature. Bacteria can also conserve energy by reducing metals, such as reduction of Fe(III) coupled to the oxidation of organic matter. Biological processes also mediate the dissolution of sulfides under acid mine drainage conditions, and there are a diversity of acidophilic (pH <3) metal sulfide-oxidizing microorganisms. The oxidation reactions of pyrite, galena, arsenopyrite and chalcopyrite are discussed in detail before considering how toxic metals may be bound to the surfaces of sulfides such as pyrite and mackinawite. The applications of sulfide bacterial redox processes in clean technologies, such as bioleaching and biomining, are discussed briefly.