Recently, the control of air pollution caused by industry and exhaust gases from automobiles becomes a critical problem. The gaseous pollutants released from various industries such as power generation plants, automotive, aerospace, glass, ceramic, petrochemical, food processing etc., spread rapidly over the large area within a short time and causes atmospheric pollution which has direct adverse effect on the human health, acid rain, ozone layer depletion, greenhouse effect and global warming. Therefore, gas sensors are increasingly demanded for the detection of concentration of a specific gas among the mixture of various gases. The metal oxide semiconductor based gas sensor is one of the widely investigated groups of gas sensor due its simplicity, ease of production, low cost and capability of detecting the large number of toxic and volatile gases. The present book summarizes the detection of hazardous gases using without and with Pd-sensitized MoO3, V2O5 and MoO3-V2O5 compositethin films.