An analytical expression for the bidirectional reflectance field of a vegetation canopy is derived from physical and geometrical considerations of the transfer of radiation through a porous medium. The reflectance pattern is shown to depend explicitly on the optical properties of the scatterers (for example, leaves), and on the structural parameters of the canopy, such as the statistical distribution of the orientation of these scatterers, the leaf area density, the size of the scatterers and their interspacing. This theory provides a simple and accurate way to understand the anisotropy of the radiation field over a vegetated surface. It can be useful for modeling applications (for example, the albedo is a by-product which can be numerically estimated), as well as for extracting some of the structural and physical properties of the surface. These applications are discussed in the accompanying paper (Pinty et al., this issue). ¿ American Geophysical Union 1990