The boom of industries has demanded the necessity of the gas sensing devices. This catches the attention of researchers towards the gas sensing materials. Semiconducting metal oxides hold the property of changing its electrical or optical parameters in adsorption/desorption of gases, which can play a crucial role in the detection of gases. To augment the properties of mixed metal oxides are used, as they tend to be complementary in their properties. This paper critically reviewed the metal oxide gas sensors and the supremacy of the heterojunction gas sensor over single metal oxide gas sensor and the sensing mechanism of metal oxide gas sensors.