The sensitivity of microwave emission to biomass and biophysical properties of crops and forests has been proved in several ex

The sensitivity of microwave emission to biomass and biophysical properties of crops and forests has been proved in several experimental and theoretical studies. Experimental investigations on microwave emission from crops carried out since early 1980s by using ground based and airborne sensors demonstrated that useful information on vegetation covered soils can be obtained by combining brightness temperatures, measured at two or more microwave frequencies and polarizations. At the lower frequencies of microwave band (around 1-3 GHz), and for small incidence angles, emission from a vegetation-covered soil is mostly influenced by soil moisture, whereas, at higher frequencies, the contribution from vegetation to the radiation is significant and the emission can be related to vegetation type and plant water content. More recently microwave radiometers on satellites, such as the recently launched AMSR-E have shown a great potential in global monitoring of natural vegetation, and a significant improvement is expected by the enhanced performances of the new generation sensors such as the planned SMOS and HYDROS. In the same time of experimental activities theoretical models were developed to interpret the observed relations between radiometer observations and vegetation and soil parameters. These models included semi-empirical approaches based on regressions obtained from radiometer observations and on the simple single scattering continuous medium radiative transfer theory as well as more sophisticated physical models based on calculating electromagnetic fields on vegetation constituents assuming either single or multiple scattering. This paper offers a review of recent achievements in both experimental and theoretical research in this field.