During the past three decades, gas sensors based either on the surface characteristics or the bulk electrolytic properties of ceramics, have been the subject of extensive research and development. The application of these sensors range from air‐to‐fuel ratio control in combustion processes such as in automotive engines and industrial furnaces to the detection of leakage of inflammable and toxic gases in domestic and industrial environments. While the solid‐state physical sensors, measuring pressure, temperature, and other physical parameters have been commercially successful, less success has been achieved by their chemical analogs, to measure moderate to very low concentrations of gases of importance. These gases include: , , , , , , propane, methane, ethanol, and so on. The semiconductor‐based chemical sensors owe their popularity to their small size, simple operation, high sensitivity, and relatively simple associated electronics. However, most of them still suffer from nonselectivity. They also have poor shelf‐life and are relatively less stable at higher temperatures. The sensing characteristics and performance of some of the solid‐state gas sensors are reviewed in this paper, together with their sensing mechanism, which still is a gray area and has not been fully understood.