Publisher Summary Rare-earth-containing mixed oxides are widely used in many engineering, electromagnetic, and electronic applications. Among these materials, perovskite-type oxides with the general formula ABO3 are the most widely studied. The ideal perovskite structure is cubic, with the B (smaller cation) in six-fold coordination and the A (larger cation) in twelve-fold coordination with the oxygens. Although there are some limits regarding the size of the cations that can fit the structure, they are wide enough as to allow the incorporation of a large variety of elements. Typical A elements are rare earths, alkalines, and alkaline earths, while the B sites are usually occupied by transition metals. Both anion and cation vacancies occur in perovskite structures. The former are more common than the latter. Vacancies are assimilated into the structure resulting in supercells of the basic network. Anion excess has been reported in several systems. Cation vacancies occur more often than B vacancies. One important driving force is the ability of these oxides to accommodate a large variety of A and B cations with basically the same crystalline structure. This series of isomorphic solids exhibits a wide range of electric, magnetic, optic, and catalytic properties that are readily applied in various commercial devices.