The “manganese-oxidizing group” is a phylogenetically diverse assemblage, which is characterized by the ability to catalyze the oxidation of divalent, soluble Mn(II) to insoluble manganese oxides of the general formula MnOx (where X is some number between 1 and 2). This results in the accumulation of conspicuous and easily detectable extracellular deposits of insoluble brown or black manganese oxides. Many different organisms have the ability to catalyze Mn oxidation, including a diverse array of bacteria, fungi, algae, and even eukaryotes (Ghiorse, 1984b). Among the prokaryotes, the ability to oxidize Mn is also quite widespread (Ehrlich, 1981; Ghiorse, 1984b, 1988; Marshall, 1979; Nealson, 1983); included are members of many phylogenetic and physiological groups: e.g., cyanobacteria, a diversity of heterotrophic rods and cocci, the sheathed (Leptothrix-like) and budding (Hyphomicrobium-like) bacteria, some purported autotrophic strains related to Pseudomonas species and the still-controversial Metallogenium group. The anaerobic lactobacilli, which utilize the Mn oxidation reaction as a protection against oxygen toxicity (Archibald and Fridovich, 1981, 1982) are not included, as they do not precipitate extracellular Mn oxides, but rather accumulate millimolar levels of protein-associated Mn in the cytoplasm. This chapter focuses on the process of Mn oxidation and also considers why so many bacteria have been identified as Mn oxidizers. It also offers suggestions that may help to clarify this complex area. Since there is no evidence of any advantage that Mn oxidation confers on bacteria, one might well ask the reason for the widespread distribution of this trait. The answer may lie in the Mn oxidation reaction itself. Under the conditions characteristic of most of the environments in which microbes are abundant, Mn is a very active element. Some critical features of Mn chemistry are summarized in Fig. 1 and are also discussed in more detail elsewhere (Ghiorse, 1988; Mulder and Dienema, 1981; Nealson et al., 1988, 1989; Pankow and Morgan, 1981). The oxidation of Mn(II) to Mn(IV) is thermodynamically favored under aerobic conditions, with a negative free energy of approximately 16 kcal/mol (Stumm and Morgan, 1981; Ehrlich, 1981; Nealson et al., 1988). However, the large activation energy of Mn(II) oxidation renders Mn(II) very stable in most aquatic environments (Stumm and Morgan, 1981). The activation energy barrier can be overcome by raising the pH (see Fig. 1) or by the addition of Mn-binding components, including Mn oxides themselves, which are excellent chelators of Mn(II) (Stumm and Morgan, 1981). The catalysis of Mn(II) oxidation by Mn oxides (autooxidation) makes it difficult to distinguish between chemically and microbially catalyzed Mn oxidation, especially in natural environments where organic chelators and Mn oxide particles may be abundant. Mn is, therefore, an element whose distribution and chemical speciation is kinetically controlled, thus allowing for the intervention of microbes and microbial products into the system. Some of the ways in which microbes might oxidize Mn(II) are shown in Table 1. If the pH or Eh of the environment is raised, if oxidants are produced by cells, or if binding of Mn(II) occurs so as to lower the activation energy, Mn(II) oxidation can rapidly proceed. With this in mind, it is not surprising that so many different bacteria have been identified as Mn(II) oxidizers, since the mechanisms of Mn oxidation are quite diverse (Ghiorse, 1988; Nealson et al., 1988, 1989). A true understanding of the “Mn-oxidizing bacteria” will likely await the time when it is possible to identify those reactions that confer some advantage to the bacteria and to disregard those that occur simply because of the dynamic chemistry of Mn(II). With regard to this, some of the recent studies of the mechanism of Mn(II) oxidation by cells, which include the isolation of Mn(II)-binding proteins (both intra- and extracellular) and polysaccharides, are particularly encouraging (Ghiorse, 1988; Nealson et al., 1989).