This chapter presents an in-depth review of rare-earth-doped metal oxide nanostructures with an emphasis on their distinct optical, electrical, and magnetic properties. The chapter discusses how the introduction of rare-earth elements such as lanthanum, cerium, and europium changes the host material properties, which results in increased functionality across different applications. The chapter outlines the specific effects of a single rare-earth dopant on typical metal oxides like ZnO, TiO2, and SnO2. In addition, it elaborates in detail the novelty and applications of these materials in three main fields: optoelectronics, including LEDs, lasers, and solar cells, catalysis, for environmental cleanup and chemical reactions, and energy storage devices, specifically batteries, supercapacitors, and fuel cells. The final section delineates the current challenges and proposes avenues for future research, stressing the importance of interdisciplinary collaborations in order to maximize the potential of these multifunctional materials.