Many phenological models use ambient air temperature to estimate phenological stages during current and projected future climate conditions. However, the difference between ambient air temperature and plant-canopy temperature biases such estimates. Therefore, consideration of how energy balance factors affect evapotranspiration leads to more realistic plant-canopy temperatures, and better prediction of plant phenological development. Evapotranspiration (ET) has a big impact on the relationship between plant-canopy and air temperature, so awareness of ET facilitates understanding of temperature based phenological models, their limitations, and possible changes in response to climate change. This chapter presents information on the estimation of reference ET (ET o ), applying crop coefficient (K c ) values to determine well-watered crop ET (ET c ), and assessing water stress effects on crop ET. It also discusses how to account for water stress effects to determine actual crop ET (ET a ) and it presents some of the problems associated with estimating the ET of natural ecosystems. The difference between plant-canopy and air temperature is shown to depend on energy balance factors that affect ET. The temperature relationships vary spatially and with time, and therefore using air temperature based degree day models can lead to errors when predicting plant phenological development, which depends on plant-canopy temperature. It is shown that the ET effect on temperature will likely decrease the accuracy of plant phenology models as climate changes.