Terrestrial biosphere models scale physiological and biophysical processes such as photosynthesis, stomatal conductance, and energy fluxes from individual leaves to the entire plant canopy. Critical to this is an understanding of leaf gas exchange, plant hydraulics, radiative transfer, and a theory and numerical parameterization to scale over all leaves in the canopy. This chapter focuses on the latter requirement and considers how to scale leaf fluxes to the canopy. Three general approaches to do so treat the canopy as: analogous to a big leaf with a single flux exchange surface without vertical structure; a dual source with separate fluxes for vegetation and soil; and in a multilayer framework in which the canopy is vertically structured and fluxes are explicitly resolved at multiple levels in the canopy.