Crystal structure and important functions of inorganic perovskite oxides are introduced. Perovskite oxides comprise large families among the structures of oxide compounds, and several perovskite-related structures are currently recognized. Typical structures (ABO3) consist of large-sized 12-coordinated cations at the A-site and small-sized 6-coordinated cations at the B-site. Several complex halides and sulfides and many complex oxides have a perovskite structure. From a variety of compositions and structures, a variety of functions are observed in perovskite oxides. In particular high electronic conductivity, which is at a similar level as metal, and surface activity to oxygen dissociation, are highly attractive in this oxide. Perovskite oxide is now widely used for solid oxide fuel cells.