Terrestrial biosphere models characterize ecosystems by features that control biogeochemical cycles and energy, mass, and momentum fluxes with the atmosphere. These include the following: leaf area index and its vertical profile in the canopy; the orientation of leaves, or leaf angle distribution; the vertical profile of leaf mass and leaf nitrogen in the canopy; the profile of roots in the soil; the size structure of plants; and the distribution of carbon within an ecosystem. This chapter defines these descriptors of ecosystems.