The surface conductivity of metal oxide based gas sensors are influenced by the composition of gases or gas/air mixture in the gas sensing environment. Native metal oxide based gas sensors like SnO2, ZnO and TiO2 have been exploited in the recent past to study their sensing abilities and selectivity towards various gases. The research carried in the recent years are aimed at adding variety of dopants mostly metal based dopants with the metal oxide semiconductor based gas sensors. The sensing mechanism involves small quantities of reducing or oxidizing gases interacting with the metal oxide sensing layer by means of chemisorption or physisorption. The sensing mechanism depends on reversible adsorption or desorption processes of the gas at semiconductor surfaces. The sensing layers are tailored made to be selective towards different gases by selecting dopants, optimal sensor temperatures, catalysts, synthesis and film fabrication techniques. Metals are doped in metal oxide primarily to enhance their conducting ability for a particular level of gas sample there by increasing the sensitivity of the metal oxide gas sensor. More over the doping also increases the selectivity of the metal oxide gas sensors towards a particular gas. The process of doping mostly involves replacing a metal atom in the crystal lattice of the metal oxide with the dopant. This leads to the change in the sensing layer resistance by several orders of magnitude. The review aims at exploring the dopants added to metal oxide semiconductor materials which can be used to sense variety of gases.