<p>Manganese (Mn) oxides, which originate from oxidative precipitation of soluble Mn(II), are a range of phases with different structural arrangements of MnO₆ octahedra. These phases are widely distributed across diverse environmental contexts, often with relatively low abundance. Nonetheless, owing to variable oxidation states of Mn and the potent oxidative and adsorptive capacities of Mn oxides, the influences of Mn oxides on the cycling of contaminants and nutrients considerably exceed its own abundance. This review aims to synthesize current knowledge on the origin, structure, and reactivity of Mn oxides. First, the biotic and abiotic origins as well as the structures of common layered and tunnel Mn oxide phases are described. Subsequently, the reactivity of Mn oxides towards redox-insensitive and redox-sensitive metals and metalloids is discussed. In particular, the intricacies of the redox interactions between Mn oxides and metal(loid)s are illustrated with two main examples: (1) arsenic, a pervasive&#xa0;global groundwater problem, and (2) chromium, a serious pollutant due to wide industrialization. The influences from the structural nature of Mn oxides and from typical environmental variables on the interactions between Mn oxides and metal(loid)s are also summarized. Finally, some future research directions in this field are outlined.</p> Graphical Abstract <p></p>