High-entropy oxides (HEOs), a subclass of high-entropy materials (HEMs), represent a transformative approach to the design of functional ceramic systems. By incorporating five or more metal cations in near-equimolar ratios into a single-phase matrix, HEOs exploit configurational entropy to suppress phase separation and stabilize novel crystalline structures. This chapter provides a comprehensive overview of HEOs, detailing their thermodynamic foundations, synthesis techniques, crystal structures, physicochemical properties, and diverse applications in energy storage, catalysis, electronics, and thermoelectrics. The chapter further delves into the remarkable electrochemical, catalytic, magnetic, thermoelectric, and electronic properties that emerge from compositional complexity and the strategic roles of oxygen vacancies, aliovalent doping, and defect engineering. Key challenges in modeling, compositional design, characterization, and scale-up are critically assessed. Finally, the chapter outlines promising future directions for expanding the HEO concept into mixed-anion systems and multifunctional devices, emphasizing the need for synergistic integration of experimental and computational approaches to accelerate discovery and application.