In this review, the literature on surface aerators is critically analysed. In a number of studies, the critical speed for the onset of surface aeration has been correlated with the geometry of the system and the operating conditions. The studies on the rate of surface aeration, gas hold-up, mass transfer coefficients are inadequate for the design and scale-up. A comparison of the available designs, in terms of gas hold-up, and mass transfer coefficient is presented. A mathematical model has been proposed for surface aeration. Guidelines have been given for the selection of impeller configuration and overall design of surface aerators. Suggestions have been made for future studies in this area.