Evapotranspiration (ET) is the total amount of water lost by plant transpiration and soil evaporation. Since about 90% of precipitation received in semi-arid and sub-humid climates is lost to the atmosphere through evapotranspiration, accurate estimations of ET are of crucial need for climate studies, weather forecasts, hydrological surveys, ecological monitoring, and water resource management. This book reviews research on new developments in all areas of evapotranspiration application. It examines the evapotranspiration processes under different environmental and climatic conditions as well as the different methods for modeling and measurement of evapotranspiration at both local and regional scales.