Gas sensing technologies have become integral to diverse fields such as environmental monitoring, industrial safety, and healthcare diagnostics. Among various sensor materials, metal oxide semiconductors (MOS) are widely used due to their cost-effectiveness, sensitivity, and ease of fabrication. Recent advancements have shown that doping these MOS materials with rare earth elements (REEs) significantly enhances their gas sensing performance. REE doping improves key attributes like surface reactivity, electron mobility, and oxygen ion exchange, resulting in greater sensitivity and selectivity for target gases. Enhanced catalytic activity, increased surface basicity, and the ability to inhibit electron–hole recombination further contribute to improved sensor response. Rare earth-doped MOS sensors have demonstrated efficient detection of gases such as ethanol, acetone, ammonia, and formaldehyde at lower operating temperatures, with faster response and recovery times. Moreover, the integration of REEs supports the development of self-powered and miniaturized sensing devices. This chapter explores the mechanisms, material modifications, and performance benefits associated with rare earth-doped MOS nanostructures, offering insights into their growing importance in next-generation gas sensing technologies.