There is always an ever-increasing demand of gas sensors widely being used in detection of feeble concentrations of combustible, highly explosive, and toxic gases, along with detection of environmental pollution. The importance of these sensors can be felt in various areas those including detection of various gases, humidity, and moisture in the environment around us, field of medicine, agriculture, and in industrial processing. Having low energy consumption, these semiconducting gas sensors have paved the way for single gas applications and possess remarkable feature of tunable sensitivity with an ability of discovering the concentrations of particular constituents. Besides the above-mentioned advantages, many disadvantages like high sensitivity with low selectivity toward various gases can be taken into account. However, by a proper choice of sensing materials and using multisensor arrays, the problem of poor precision and selectivity can be easily overcome. An electronic nose system comprises multiple sensor array components that can respond to a particular odor using an electronic fingerprint. This chapter gives an overview of various metal oxide-based gas sensors especially thin film transistor-based gas sensors with their operational mechanism. These have intriguing operational advantages over the conventional sensors which are mainly two terminal resistive types and are thus explicitly reviewed in the chapter.