The scattering behaviour of the earth's surface at radar wavelengths can provide useful information about many natural processes. Since most applications of remote sensing require large areas to be surveyed, HF radars provide the only useful ground-based systems. These are used for sea sensing. Extensive coverage can be provided by air-borne platforms. Scatterometers mounted on helicopters or aircraft, and air-borne side-looking radars and SARs are employed around the world for a wide range of applications. Perhaps the most exciting prospect is the new generation of space-borne radar instruments, which promise to be a major source of information on global-scale processes. Weather conditions have little effect on these instruments at the range of wavelengths employed (though ionospheric effects have to be allowed for in altimetry and SAR operation). This will allow reliable gathering of surface information, which cannot be guaranteed at optical wavelengths. Radar instruments can also provide information, such as the global wind field over the oceans, that is not available by other means. Much of this information, such as the global wind field or mean sea height, can be gathered at comparatively low spatial resolutions by scatterometry or altimetry (though high-resolution altimetry is attracting much current interest, and we can expect significant progress in the range of problems to which altimetry can be applied). For applications requiring high spatial resolution, SAR provides a possible answer, at the expense of increased system complexity (and cost!) and problems of data interpretation in the presence of speckle. With the launch of ERS-1 and other space-based radars throughout the 1990s, this decade should see major advances in understanding these techniques and their application to monitoring the earth's environment.