The book emphasizes on the development of gas sensors fabricated using thin films of PPy, ZnO nanoparticles, PPy-ZnO nanocomposite and PPy-ZnO-CSA for oxidizing and reducing toxic gases recognition. Gas response curves of materials were analyzed to find selectivity of films towards particular gas and for evaluation of other parameters mentioned above. The gas sensing performance at different concentrations of selective gas is carried out at room temperature and the performance of the sensors is evaluated in terms of their selectivity, sensitivity and response-recovery times. Lastly, Gas sensing mechanism and physical model for the same wherever possible is discussed.