Reference evapotranspiration (ET_o) is considered an essential factor in determining the meticulous estimation of crop water requirement and effective irrigation scheduling. The accurate estimation of crop water requirement is of critical importance to minimize over and under irrigation problems. Several empirical/semi empirical equations have been developed in the past to quantify ET_o. The Penmen-Montieth equation (FAO-PM56) has been globally accepted for estimation of ET_o but certain limitations were found to its implementation. The use of soft computing models in estimation of ET_o has received enormous interest in recent decade. Many studies have been reported in the literature to apply soft computing models on the improvement of ET_o estimation. This study intended to review these studies on basis of accuracy, structure and its flexibility/usefulness, and also made some possible suggestions for future research in this domain.