A novel class of functional materials known as high-entropy oxides (HEOs) is defined by the nearly equimolar addition of five or more metal cations to a single-phase oxide structure. The formation of single solid solutions that would typically result in phase separation is made possible by this compositional complexity, which is stabilized by configurational entropy. In order to achieve previously unheard-of material properties, such as enhanced ionic conductivity (0.01–0.1 S/cm at 800 °C), distinct magnetic behaviors, superior thermal stability, and remarkable catalytic activity, HEOs intentionally use disorder. Conventional solid-state reactions are the most basic synthesis methods; more sophisticated approaches include sol–gel processing, combustion synthesis, hydrothermal methods, and pulsed laser deposition for thin-film deposition. The entropy-stabilized (Mg, Co, Ni, Cu, Zn)O is the standard system of these materials, which crystallize in a variety of structural types such as rock salt, perovskite, spinel, and fluorite structures. Energy storage (batteries and supercapacitors), solid oxide fuel cells, environmental catalysis, gas sensing, photocatalysis, and high-temperature structural applications are just a few of the many applications where HEOs show great promise. This chapter explains how HEOs are positioned as transformative materials for next-generation sustainable technologies because of their high defect densities and compositional tunability, which allow property optimization for particular technological requirements.