Extension of the bulk surface energy balance to include vegetation involves formulation of leaf fluxes. Energy from the net radiation absorbed by a leaf is stored in biomass or is dissipated as sensible and latent heat, and the balance of these fluxes, as influenced by prevailing meteorological conditions, leaf biophysics, and leaf physiology, determines leaf temperature. This chapter develops the biophysical foundation and mathematical equations to describe leaf temperature and the leaf energy budget. A critical determinant of leaf fluxes and temperature is leaf boundary layer conductance, which depends on wind speed and leaf size.