Rare earth element (REE)-doped metal oxide nanostructures have surfaced as a prominently advanced class of next-generation functional materials for the new advanced technologies. The chapter provides a meticulous review of recent advancements in this rapidly developing field, emphasizing how REE doping and co-doping offer opportunities for precisely tailoring electrical, optical, magnetic, and catalytic characteristics through defect engineering, mixed valence states, and carefully designed nano-architectures. This is complemented by a display of the most significant developments in co-doping applications, nano-architectural and surface modifications, as well as their integration into various flexible, wearable, and next-generation electronic devices. In addition, the chapter points out the decisive contribution of modern sophisticated in-situ and operando characterization techniques to understanding the real-time material electronic, structural, and chemical transformation phenomena, hence informing a more rational material design. Again, the important challenges posed by issues such as scalable synthesis, material stability, and economic viability are given a critical appraisal. The chapter provides a glance at the future of scientific endeavour, especially focusing on the sustainable synthesis strategies, optimization through machine learning, and their applications to energy storage, catalysis, optoelectronics, environmental remediation, and biomedicine. The chapter underscores the transformative potential of the REE-doped metal oxides in addressing the upcoming demands of new technology and global sustainability.