Manganese-based oxides are attracting extensive interest in electrochemical energy storage and conversion devices, because of their wide compositional and structural diversity, high elemental abundance, and low cost. However, the structure-properties relationship of Mn-based oxides remains elusive and their electrochemical activities require further improvement. Recently, we have controllably synthesized a variety of manganese oxides (e.g., binary oxides of MnO₂ and Mn₂O₃, multimetallic composite manganese oxides of Co-Mn-O and Ca-Mn-O) with different phases, sizes and morphologies. We have investigated the catalytic properties of these oxides for the oxygen reduction/evolution reactions and applied them as cathode electrocatalysts for metal-air batteries. Our results indicate that tuning the Mn oxidation states and material microstructures play important roles in enhancing the performance of manganese oxides. The presence of multivalent Mn and the introduction of oxygen deficiency are found to favor oxygen elecstrocatalysis in both aqueous and aprotic media, while the particularly designed porous electrode nanostructures benefit reversible formation of Li₂O₂ at low overpotentials in lithium-oxygen batteries. To further improve the electrochemical performance, we have prepared manganese oxide-based hybrid nanocomposites with integrated functional components or substrates such as carbon, platinum, and nickel.