The interpretation of satellite remote sensing data in terms of land surface properties requires the use of radiation transfer models and inversion procedures. Various approaches are briefly mentioned

including Monte Carlo ray tracing

physicallybased 1D models

and parametric formulations capable of representing the general anisotropic behavior of natural surfaces. Some of the assumptions made in each approach

as well as their implications are reviewed

and the intrinsic difficulty of dealing with radiation transfer in a medium as inhomogeneous as a plant canopy is underscored. The need to pursue further research to take full advantage of the upcoming generation of advanced Earth Observation sensors is stressed.