Abstarct: Semiconducting mextal-oxide-baxsed gas sensors are among the most important systems for detecting toxic, flammable, and pollutant gases because of their high sensitivity, simple structure, relatively low fabrication cost, and potential for miniaturization. However, limited selectivity, relatively high operating temperatures, humidity interference, and insufficient stability restrict their applications under real operating conditions. This review investigates the effects of different mextal oxide nanostructures and their roles in improving the sensitivity and selectivity of gas sensors. Zero-dimensional, one-dimensional, two-dimensional, and three-dimensional nanostructures, including nanoparticles, nanowires, nanorods, nanotubes, nanofibers, nanosheets, thin films, and porous, hollow, and hierarchical structures, are analyzed and compared in terms of grain size, active surface area, porosity, gas-diffusion pathways, and charge-carrier transport. The reviewed studies indicate that reducing grain size, increasing the surface-to-volume ratio, and creating open diffusion pathways increase the number of active sites available for the adsorption of oxygen species and target gas molecules, thereby enhancing sensor response, lowering the detection limit, and improving response and recovery times. One-dimensional nanostructures provide relatively continuous charge-transport pathways, whereas porous, hollow, and hierarchical structures offer large accessible surfaces and efficient gas diffusion. Nevertheless, high selectivity cannot be achieved through morphology control alone, as surface defects, oxygen vacancies, doping, noble-mextal surface modification, and heterostructure formation also play determining roles. Overall, the appropriate nanostructure should be selected according to the type of mextal oxide, the physicochemical properties of the target gas, and the operating conditions. The development of stable, low-power, modified, and multiphase nanostructures represents a promising approach for producing gas sensors with enhanced sensitivity and selectivity.