Rare earth elements play a pivotal part of tailoring the capabilities of metal oxides, significantly enhancing their utility across a broad spectrum of technological applications. This chapter explores how REE incorporation modifies the optical, electrical, and structural attributes of prominent metal oxides such as ZnO, NiO, SnO2, and TiO2. Through doping, REEs can adjust optical behaviors by shifting absorption spectra, inducing photoluminescence and boosting photocatalytic efficiency-traits valuable for applications in photovoltaics, optical sensors, and photocatalysis. Electrically, REEs addition influences charge carrier dynamics, modulates conductivity, and alters dielectric properties, thereby improving the performance of semiconducting materials in sensors, spintronic systems, and electrochemical platforms. On the structural front, REEs can impact crystal lattice configurations, refine grain size, and enhance thermal robustness, collectively contributing to greater mechanical integrity, catalytic reactivity, and long-term material stability.