Metal oxide semiconductor (MOS) gas sensors are widely used in environmental monitoring due to their low cost, robustness, and sensitivity. They are particularly effective for detecting gases like CO, NOx, NH3, and CO2. This review discusses the principles of gas sensing, the impact of surface structure on sensor response, and recent advancements in sensor design and fabrication. The response of MOS sensors is based on changes in charge carrier concentration due to gas interaction with the metal oxide surface. For example, reducing gases increase conductivity in n-type semiconductors, while oxidizing gases decrease it in p-type semiconductors. The response is influenced by factors such as grain size, surface area, and the presence of interfering gases like ozone, water, and volatile organic compounds. Recent advances include the use of selective zeolite layers, new perovskite-type materials, and innovative chemical vapor deposition techniques to improve sensor specificity and efficiency. Fabrication methods such as screen printing, chemical vapor deposition (CVD), spray pyrolysis, sol-gel, and physical vapor deposition (PVD) are discussed, with each method having its advantages and limitations. For CO detection, SnO2 is the most commonly used material, with optimal performance at temperatures around 250°C. For CO2 detection, LaOCl has shown promising results, with high sensitivity and lower operating temperatures. However, challenges remain in achieving reliable detection in the 500–2000 ppm range. For NOx detection, WO3-based sensors have been developed, with grain size and annealing temperature affecting sensitivity and response time. Overall, while MOS sensors offer cost-effective solutions, further research is needed to improve their performance, especially for CO2 detection, where traditional doping methods have proven insufficient.Metal oxide semiconductor (MOS) gas sensors are widely used in environmental monitoring due to their low cost, robustness, and sensitivity. They are particularly effective for detecting gases like CO, NOx, NH3, and CO2. This review discusses the principles of gas sensing, the impact of surface structure on sensor response, and recent advancements in sensor design and fabrication. The response of MOS sensors is based on changes in charge carrier concentration due to gas interaction with the metal oxide surface. For example, reducing gases increase conductivity in n-type semiconductors, while oxidizing gases decrease it in p-type semiconductors. The response is influenced by factors such as grain size, surface area, and the presence of interfering gases like ozone, water, and volatile organic compounds. Recent advances include the use of selective zeolite layers, new perovskite-type materials, and innovative chemical vapor deposition techniques to improve sensor specificity and efficiency. Fabrication methods such as screen printing, chemical vapor deposition (CVD), spray pyrolysis, sol-gel, and physical vapor deposition (PVD) are discussed, with each method having its advantages and limitations. For CO detection, SnO2 is the most commonly used material, with optimal performance at temperatures around 250°C. For CO2 detection, LaOCl has shown promising results, with high sensitivity and lower operating temperatures. However, challenges remain in achieving reliable detection in the 500–2000 ppm range. For NOx detection, WO3-based sensors have been developed, with grain size and annealing temperature affecting sensitivity and response time. Overall, while MOS sensors offer cost-effective solutions, further research is needed to improve their performance, especially for CO2 detection, where traditional doping methods have proven insufficient.