The mechanisms of pyrite formation are reviewed. Advances since 1994 in our understanding of the mechanisms and rate of pyrite formation, the role of bacteria in the formation of pyrite, framboid formation, and incorporation of impurities into pyrite are emphasized. Both field studies as well as laboratory studies designed to better represent natural environments have provided significant new insights. Field studies suggest that hydrogen sulfide can sulfidize amorphous FeS and form pyrite. The reaction rate as determined in the field is orders of magnitude slower than in laboratory experiments, and questions remain about the role of the FeS surface and the electron acceptor involved in the conversion. It is also becoming increasingly clear that sulfate-reducing bacteria play a more important role than simply providing hydrogen sulfide for the reaction. Experiments with in vitro cultures demonstrate the role of cell walls in directing and promoting the precipitation process. Synthesis of nanoscale pyrite, trace element incorporation, and formation of defects in pyrite are new research directions that are examined.