The purpose of this article is to give a possibly subjective critical overview on chemical gas sensors, focusing on oxide-semiconductor-based devices. The first part of this work covers - starting from the sensing principles - the elementary physical and chemical steps used in the process, as well as the materials used. The depth of understanding of the elementary processes is treated as a function of the experimental methods applied today. The importance of the layer thickness, grain size and macro and microstructure is considered. The two aspects of gas/solid heterogeneous catalysis are discussed: a./ for a chemist, the selectivity in the gas phase is important, b./ for the sensor specialist, the changes in the solid phase during the gas-solid interaction, as well as the selectivity in the analyte, are of importance. As a next step, the experimental aspects of the field are examined. The methods of improving the “3S” properties of a sensor are analysed, mainly from the point of view of materials science. Several less-common principles and new ideas and technical tricks suitable for detecting gases or improving the sensor operation are briefly described. In the above two parts, attention has been paid to the unsolved problems of modern sensor science, such as avoiding cross effects, the undesired damage of the sensor material (corrosion), reproducibility of the sample preparation or insufficient stability. In the last part, the discussed problems, where possible, are illustrated by examples of our own: a./ The phase transitions in the solid are of special interest. In our experience, the temperature and extent of the phase transitions can be influenced by the presence of another solid or gas component. Here the effect of water is treated. b./ The authors present their latest results obtained in the detection of CO by Pt and Pd modified SnO2 using work function measurements.