Gas sensors has been currently in great demand because of serious concern over environmental pollution and public health considerations resulting from tremendous growth of industrialisation. Concurrently, there have been continuous efforts to obtain sensors with improved performance. The performance of any solid-state electrochemical gas sensor has been always rated on its response time, thermodynamic stability, operating temperature, gas sensing ability, sensitivity and gas concentration range, which is to be sensed. This article reviews the factors contributing towards a gradual development of electrochemical solid-state gas sensors in terms of a continuous tailoring of its two basic components, i.e. solid electrolyte and reference electrode.