The main semiconductor oxides for gas sensing have been examined, describing the synthesis processes and the morphological and structural properties. The powders have been deposited as thick films through screen-printing technique to achieve gas sensors. The sensing layers have been studied with regard to semiconductor behaviour, surface barrier potential heights and gas sensing properties. The interaction of humidity with semiconductor oxides has been considered with the aim to compensate, using an algorithm, its effect in gas detection. Finally, nitrogen oxides monitoring, contemporaneously performed in several sites of Italy, has been described.