Actual and maximum rates of evaporation (E) and evapotranspiration (ETevapotranspiration (ET)) are important to the operation of atmospheric process models and for hydrologic and agricultural modeling. Because rates of evapotranspiration are limited by both the available energy at the surface and the availability of water, a variety of techniques can be used for estimation. The near-maximum ET under nonlimiting water availability can be closely approximated by the reference ET concept using near-surface observations of air temperature, humidity, wind speed, and solar radiation via the Penman–Monteith or a similar method. The determination of actual rates of ET when water is limiting demands a more complex approach, and often requires daily (or even more frequent) water balance data for the upper soil layers. An alternative is to measure the actual ET using micrometeorological techniques such as the eddy-covariance and Bowen ratio methods. The application of standardized calculations for the reference ET is discussed, as are iterative surface energy balance–aerodynamic combinations, which are useful in conditions where water is limiting.