The transfer of energy and water, as well as gaseous components, in the soil–plant–atmosphere system plays a crucial role in many processes involved in climate change. The surface energy balance drives both evapotranspiration (closely linked to the CO2 flux) and sensible heat flux. By providing direct information on heat dissipation efficiency, surface temperature appears to be a key variable. In particular, a lot of work has been conducted on estimating evapotranspiration from thermal infrared (TIR) remote sensing measurements, in order to derive information relevant to agriculture (monitoring plant growth, detection of water stress, crop yield forecasting, etc.) or hydrology (water cycle monitoring, catchment water budget, etc.). This chapter first very succinctly presents the relationship between surface temperature and the surface energy balance, providing the reader with reference publications for further reading. This is followed by a review of the data available in the thermal infrared (TIR) domain and an analysis of various sources of uncertainty affecting surface temperature measurements and their impact on the final accuracy. The different methods for estimating and spatializing actual evapotranspiration (AET) are then discussed. Finally, practical applications of TIR information other than AET are briefly mentioned.