Mixed-anion compounds, in which multiple anions such as O2−, N3−, and H− are contained in the same compound, have recently attracted attention. Because mixed-anion compounds have a unique crystal structure with multiple anions coordinated to cations, materials with fundamentally new and innovative functions are expected to be developed for various chemistry and physics applications, including catalysts, batteries, and superconductors. In this Account, recent progress in the development of new mixed-anion compounds by the MEXT mixed-anion project is described, with emphasis on results related to the development of materials used as photocatalysts/photoelectrodes, phosphors, secondary battery components, conductors, and thermoelectric materials.