Accurate modelling of evapotranspiration (ET) is required to predict the effects of climate and land use changes on water resources, agriculture and ecosystems. Significant progress has been made in estimating ET at the global and regional scale. However, further efforts are needed to improve spatial accuracy and modeling capabilities in alpine regions (Brooks & Vivoni, 2008b). This chapter will point out the components of the energy budget neededto model ET, to discuss the fundamental equations and to provide an extended review of theparametrizations available in the hydrological and land surface models literature. The second part of the chapter will explore the complexity of the energy budget with special reference to mountain environments.