The book emphasizes on the development of gas sensors fabricated using thin films of PPy, PPy-CSA, -Fe2O3, PPy/ -Fe2O3 hybrid nanocomposite and CSA doped PPy/ -Fe2O3 hybrid nanocomposite for the detection of various oxidizing (NO2, Cl2) and reducing gases (NH3, CH3OH, C2H5OH, H2S). Gas sensing performance at different concentrations of selective gas was carried out at room temperature and the performance of the sensors is evaluated in terms of their response, selectivity, response-recovery times, and stability. Finally, sensing mechanism for the same wherever possible is discussed with the help of impedance spectroscopy.