Manganese is an element that is relatively abundant and can be found in three oxidation states (II, III and IV) in the environment. Mn(II) is generally found in the soluble phase while Mn(IV), normally found as Mn(IV) oxide is relatively insoluble. Mn(III) is unstable in the environment and will disproportionate into Mn(II) and Mn(IV) unless it is complexed by a ligand, which allows Mn(III) to be present in the soluble phase. Mn(IV) oxide is a strong oxidant and sorbent that plays an important role in the cycling and mobility of various elements and organic compounds. In the environment Mn(IV) oxides are predominantly formed by bacteria (and fungi) through the oxidation of Mn(II) via a Mn(III) intermediate. Although the reason(s) why bacteria oxidize Mn(II) is still unknown, recent studies have revealed the nature of the enzymes that carry out oxidation of Mn(II) to Mn(IV). Studies have shown that depending on the bacterium, a multicopper oxidase and/or a heme peroxidase is involved in bacterial Mn(II) oxidation, similar to fungi. Studies also have reported the Mn oxides formed by Mn(II)-oxidizing bacteria to be layer-type birnessites. This chapter aims to introduce what is currently known about how bacteria form Mn oxides and also the structures of bacteriogenic Mn oxides.