This review discusses the gas sensing properties of metal oxide nanostructures, focusing on their sensitivity, selectivity, response speed, and the effects of particle size, morphology, and doping. Metal oxide gas sensors are widely used due to their low cost, ease of production, and compact size. However, their performance is significantly influenced by the morphology and structure of the sensing materials, which can hinder achieving high sensitivity. To overcome these limitations, various metal oxide nanostructures such as porous nanotubes, nanospheres, and nanowires have been developed. These structures offer large surface areas, numerous reactive sites, and loose film structures that are advantageous for gas diffusion. The "small size effect" is a key factor in enhancing sensor sensitivity. When the particle size of metal oxides is close to or less than double the thickness of the space-charge layer, the sensor sensitivity increases significantly. However, small metal oxide nanoparticles may compact during film coating, which is disadvantageous for gas diffusion. Doping is also an effective method to decrease particle size and improve gas sensing properties. The review highlights the importance of doping in forming p-n junctions, which can increase the depletion barrier height and improve sensor response. Porous metal oxide structures, such as mesoporous SnO₂, are particularly effective for gas sensing due to their high permeability and large surface area. These structures allow gas molecules to diffuse more easily and interact with the inner grains, enhancing the sensor's response to various gases. Porous nanowires, nanotubes, and nanosheets are also discussed, with their unique properties contributing to improved gas sensing performance. Hollow and porous nanospheres are noted for their ability to adsorb gases on both inner and outer surfaces, enhancing sensitivity and reversibility. Doping metal oxide nanostructures at the nanoscale can further enhance their gas sensing properties. For example, Cu-doped SnO₂ and Au- or Pt-doped In₂O₃ nanowires show improved sensitivity and selectivity. The review also discusses the development of new doping techniques, such as plasma-assisted strategies, to achieve uniform and dense doping of metal oxide nanostructures. These advancements are expected to lead to more sensitive and stable gas sensors in the future. The review concludes that controlling the morphology and structure of sensing materials is crucial for improving gas sensing performance, and that further research in this area is needed to develop more efficient and reliable gas sensors.This review discusses the gas sensing properties of metal oxide nanostructures, focusing on their sensitivity, selectivity, response speed, and the effects of particle size, morphology, and doping. Metal oxide gas sensors are widely used due to their low cost, ease of production, and compact size. However, their performance is significantly influenced by the morphology and structure of the sensing materials, which can hinder achieving high sensitivity. To overcome these limitations, various metal oxide nanostructures such as porous nanotubes, nanospheres, and nanowires have been developed. These structures offer large surface areas, numerous reactive sites, and loose film structures that are advantageous for gas diffusion. The "small size effect" is a key factor in enhancing sensor sensitivity. When the particle size of metal oxides is close to or less than double the thickness of the space-charge layer, the sensor sensitivity increases significantly. However, small metal oxide nanoparticles may compact during film coating, which is disadvantageous for gas diffusion. Doping is also an effective method to decrease particle size and improve gas sensing properties. The review highlights the importance of doping in forming p-n junctions, which can increase the depletion barrier height and improve sensor response. Porous metal oxide structures, such as mesoporous SnO₂, are particularly effective for gas sensing due to their high permeability and large surface area. These structures allow gas molecules to diffuse more easily and interact with the inner grains, enhancing the sensor's response to various gases. Porous nanowires, nanotubes, and nanosheets are also discussed, with their unique properties contributing to improved gas sensing performance. Hollow and porous nanospheres are noted for their ability to adsorb gases on both inner and outer surfaces, enhancing sensitivity and reversibility. Doping metal oxide nanostructures at the nanoscale can further enhance their gas sensing properties. For example, Cu-doped SnO₂ and Au- or Pt-doped In₂O₃ nanowires show improved sensitivity and selectivity. The review also discusses the development of new doping techniques, such as plasma-assisted strategies, to achieve uniform and dense doping of metal oxide nanostructures. These advancements are expected to lead to more sensitive and stable gas sensors in the future. The review concludes that controlling the morphology and structure of sensing materials is crucial for improving gas sensing performance, and that further research in this area is needed to develop more efficient and reliable gas sensors.