A review is presented of various methods to determine the evapotranspiration rate under field conditions. It is shown that, at the present time, the data which have been collected by a suitable lysimetric method are the only ones that exist in quantity and that, at the same time, can be considered as reliable.The conditions which must be met by a lysimeter installation for the accurate and representative measurement of the evapotranspiration rate are reviewed. It is shown that both the exposure as well as the moisture conditions in the soil of the lysimeter must be representative of those in the surrounding area if realistic values are to be obtained.Selected data from four locations in the Eastern United States are presented and compared. It is shown that data for grass cover under a variety of geographic and climatological conditions do not vary greatly from one another. Such dependable comparisons as are available between different crops show that the differences between corn, wheat, and meadow crops are small though not insignificant. The need for additional and adequate lysimetric measurements of evapotranspiration rates in the eastern half of the United States is discussed as well as the necessity for making pertinent meteorological measurements to aid in the generalization of the data.