Abstract This chapter gives an overview of the conduction mechanisms in semiconducting metal oxide (SMOX) sensing films and their impact on the transduction of surface chemistry into a measurable sensor signal (relative change of resistance). Following general discussion on the functioning of sensors based on SMOX porous, sensing films, modeling of the conduction in p- and n-type materials is performed. The theoretical results explain why the sensor signals are lower for p-type oxides when compared with n-type oxides. The modeling concepts are verified by applying them to experimental results obtained with sensors based on both n- and p-type SMOXs with good results. In combination with the modeling results, further experiments in more realistic conditions (exposure to CO in humid air) demonstrate that also in these conditions the switch between conduction mechanism controlled by surface depletion layer to one controlled by surface accumulation layer may occur.