The book describes how new insights about the basic understanding of semiconducting metal oxide based gas sensors can be gained by a new developed combination of theoretical modeling and experimental investigations. By applying studies under real working conditions, it is demonstrated that the surface chemistry is not that simple as anticipated and very much depends on the oxygen background. In addition, the way in which the conduction is taking place in such polycrystalline thick film layers is crucial for the magnitude of the measurable signal. The physical parameters of interest for the conduction processes in the sensing layer are the overall resistance and the corresponding band bending at the surface and the experimental relationship between them can be confronted with various conduction mechanism modeling.A change in the conduction mechanism of undoped SnO2 thick film layers is demonstarted by both, the theory and the experiments. Further, differnet properties of undoped and several doped samples can be estimated. CuO and WO3 are additionally investigated and compared to SnO2.