The information of actual areal evapotranspiration has paramount importance in hydrology, but a method easily applicable in the practice and providing acceptable accuracy for its estimation is still missing. The concept based on the complementary character of actual and potential evapotranspiration offers a promising way. The practical application of this approach requires the precise interpretation of the independent variables considered in the model. The purpose of the paper is to promote the better understanding and the wide application of this method by analysing some open questions of the complementary theory.