Surface soil moisture is a key variable to describe the water and energy exchanges at the land surface/atmosphere
interface. However, soil moisture is highly variable both spatially and temporally. Passive microwave remotely
sensed data have great potential for providing estimates of soil moisture with good temporal repetition (on a daily basis)
and at regional scale (∼10 km). This paper reviews the various methods for remote sensing of soil moisture from
microwave radiometric systems. Potential applications from both airborne and spatial observations are discussed in the
fields of agronomy, hydrology and meteorology. Emphasis in this paper is given to relatively new aspects of microwave
techniques and of temporal soil moisture information analysis. In particular, the aperture synthesis technique allows us
now to a address the soil moisture information needs on a global basis, from space instruments.