The chapter discusses the microbial dissimilatory reduction of Fe(III) and Mn(IV), which significantly influences the biogeochemical cycles of carbon and metals. Fe(III) reduction is considered the most important chemical change in anaerobic soils and sediments, potentially being the first globally significant mechanism for the oxidation of organic matter to carbon dioxide. The reduction of Fe(III) and Mn(IV) affects the distribution of iron and manganese in aquatic environments and can influence the distribution of toxic trace metals and phosphate. The chapter outlines the types of dissimilatory Fe(III) and Mn(IV) reducers, including fermentative, sulfur-oxidizing, hydrogen-oxidizing, and organic-acid-oxidizing microorganisms. Fermentative microorganisms, such as Escherichia coli and Clostridium pasteurianum, can reduce Fe(III) and Mn(IV) during anaerobic growth. Sulfur-oxidizing microorganisms like Thiobacillus thiooxidans can reduce Fe(III) to Fe(II) under aerobic conditions. Hydrogen-oxidizing microorganisms, such as Pseudomonas sp. and Shewanella putrefaciens, oxidize hydrogen to reduce Fe(III) or Mn(IV). Organic-acid-oxidizing microorganisms, like GS-15, can oxidize acetate and other organic compounds to carbon dioxide with the reduction of Fe(III) or Mn(IV). The chapter also discusses the pathways for microbial oxidation of sedimentary organic matter coupled to Fe(III) and Mn(IV) reduction. This process involves the hydrolysis of complex organic matrices into smaller components, followed by the metabolism of these components by fermentative and Fe(III)/Mn(IV)-reducing microorganisms. The chapter emphasizes that microbial oxidation of organic matter coupled to Fe(III) and Mn(IV) reduction is a more significant process than nonenzymatic reduction by organic compounds or reduced sulfur compounds. Finally, the chapter addresses the relative potential for enzymatic and nonenzymatic reduction of Fe(III) and Mn(IV). While nonenzymatic reduction mechanisms like the redox model and direct-reduction model have been proposed, evidence suggests that enzymatic reduction by specialized microorganisms is the primary mechanism. The chapter concludes by highlighting the importance of microbial Fe(III) and Mn(IV) reduction in various aquatic environments and the need to further investigate the relative contributions of abiological and biological processes.The chapter discusses the microbial dissimilatory reduction of Fe(III) and Mn(IV), which significantly influences the biogeochemical cycles of carbon and metals. Fe(III) reduction is considered the most important chemical change in anaerobic soils and sediments, potentially being the first globally significant mechanism for the oxidation of organic matter to carbon dioxide. The reduction of Fe(III) and Mn(IV) affects the distribution of iron and manganese in aquatic environments and can influence the distribution of toxic trace metals and phosphate. The chapter outlines the types of dissimilatory Fe(III) and Mn(IV) reducers, including fermentative, sulfur-oxidizing, hydrogen-oxidizing, and organic-acid-oxidizing microorganisms. Fermentative microorganisms, such as Escherichia coli and Clostridium pasteurianum, can reduce Fe(III) and Mn(IV) during anaerobic growth. Sulfur-oxidizing microorganisms like Thiobacillus thiooxidans can reduce Fe(III) to Fe(II) under aerobic conditions. Hydrogen-oxidizing microorganisms, such as Pseudomonas sp. and Shewanella putrefaciens, oxidize hydrogen to reduce Fe(III) or Mn(IV). Organic-acid-oxidizing microorganisms, like GS-15, can oxidize acetate and other organic compounds to carbon dioxide with the reduction of Fe(III) or Mn(IV). The chapter also discusses the pathways for microbial oxidation of sedimentary organic matter coupled to Fe(III) and Mn(IV) reduction. This process involves the hydrolysis of complex organic matrices into smaller components, followed by the metabolism of these components by fermentative and Fe(III)/Mn(IV)-reducing microorganisms. The chapter emphasizes that microbial oxidation of organic matter coupled to Fe(III) and Mn(IV) reduction is a more significant process than nonenzymatic reduction by organic compounds or reduced sulfur compounds. Finally, the chapter addresses the relative potential for enzymatic and nonenzymatic reduction of Fe(III) and Mn(IV). While nonenzymatic reduction mechanisms like the redox model and direct-reduction model have been proposed, evidence suggests that enzymatic reduction by specialized microorganisms is the primary mechanism. The chapter concludes by highlighting the importance of microbial Fe(III) and Mn(IV) reduction in various aquatic environments and the need to further investigate the relative contributions of abiological and biological processes.