Evapotranspiration is a function of soil, plant, and meteorological factors. Attempts to predict evapotranspiration without considering all pertinent factors can meet with only qualified success.It is found that evapotranspiration is controlled by: soil moisture tension, physiological factors, the relation of soil moisture of an irrigated area to that of its surroundings as well as purely meteorological factors of radiation, wind, air temperature, and humidity.Examples are given of the effects of these factors on measured evapotranspiration.