Manganese oxides are a uniquely diverse class of materials both structurally and chemically. Over 30 Mn (oxy)(hydr)oxide minerals are known to form via natural processes with a wide range of possible crystal structures, Mn oxidation states, and intercalated ions. An even greater variety can be synthesized with simple laboratory methods. In nature, this rich assortment of structure and ion insertion chemistry plays a vital role in regulating biological as well as geochemical cycles. It has also sparked significant interest in using Mn oxides in next generation energy storage and conversion, chemical separations, and pollution control technologies. Mn atoms can adopt oxidation states ranging from 0 to +7 depending on their environment. In solid (oxy)(hydr)oxide phases, Mn cations are generally found in the +2, +3 or +4 oxidation states, octahedrally coordinated by oxygen. However, Mn can also appear in tetrahedral coordination in phases like Mn 3 O 4 . A wide variety of Mn oxide crystal structures arises from different configurations of MnO 6 octahedra (shown in Figure ). These octahedra assemble into linear chains or 2D planes via edge sharing (i.e. adjacent Mn ions sharing two oxygen atoms). Extended crystal structures with 1D tunnels are formed when these chains attach through corner sharing oxygens, and 2D layered structures are formed when planes of octahedra stack. Common Mn oxide polymorphs include the 𝛼 (2×2 tunnels), β (1×1 tunnels), R (2×1 tunnels), 𝛿 (layered) and λ (spinel) structures (Fig. ). Amorphous Mn oxides, intergrowths (𝛾-MnO x with both 2×1 & 1×1 tunnels) and larger tunnel structures (𝜏-MnO x with 3×3 tunnels) also exist (structures not shown).