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Satellite-based models for determining evapotranspiration (ET) are now routinely applied as part of water and water resources management operations of state and federal agencies. Strengths and weaknesses of more common models are briefly described. The more dependable and universal satellite-based models employ a surface energy balance (EB) where ET is computed as a residual of surface energy. This determination requires a thermal imager onboard the satellite, which is not common. The 'CIMEC' approach ("calibration using inverse modeling of extreme conditions") used by two moderate resolution, operational models is described where CIMEC calibrates around uncertainties and biases in satellite based energy balance components. Creating maps of ET that are useful in management and in quantifying and managing water resources requires the computation of ET over monthly and longer periods such as growing seasons or annual periods. Interpolation between images from snapshot models involves treatment of clouded areas of images, accounting for evaporation from wetting events occurring prior to or following overpass dates. A technique currently used in the METRIC model for accounting for evaporation from precipitation between images is described. How the interpolation is done substantially impacts the quality and accuracy of the final ET product.