Manganese (Mn) oxides are among the most reactive minerals in the earth's surface environment, and play a significant role in adsorption, co-precipitation and redox reactions, affecting biogeochemical cycles of numerous nutrients and contaminants. In addition, manganese oxides are widely applied as absorbents, ion sieves, chemical oxidants and catalysts for environmental remediation and pollution control. There are, however, still many aspects poorly understood in terms of the environmental geochemistry of manganese oxides, such as biogenic formation mechanisms, detailed crystal structure of nanocrystalline phases, electron transfer paths in redox reactions, adsorption mechanisms of contaminants on surfaces, physiochemical factors controlling the contents of structural Mn(III) and vacant sites that largely determine Mn oxide reactivity, and the role of Mn(III) in the above processes. This book aims to bring together advances on manganese oxide environmental biogeochemistry from leading scientists in multiple disciplines, e.g., mineralogy, geochemistry, soil science and environmental engineering. A number of frontier research topics are included in the book, such as in-situ characterization of Mn oxide reactivity with As and Cr, kinetics and molecular-scale mechanisms of metal oxidation by mangnese oxides, mechanistic understanding of metal sorption through density function theory, the role of surface edge sites of birnessites in metal(loid) sorption, reactivity of natural crytomelane, pathways of phyllomanganate transformation to todorokite, water-oxidation catalysis by manganese oxides, in situ As immobilization using stabilized Fe-Mn binary oxides, and synthesis of Li ion-sieves from biogenic oxides and stability of colloidal Mn oxides. The book should be of interest to scientists and engineers in a broad range of disciplines, such as geochemistry, soil science, mineralogy, microbiology, materials science, and environment engineering, as well as graduate students who are engaged in research on manganese oxide biogeochemistry and the engineering application of Mn-bearing materials.