During the last 3 years we performed in situ backscattering measurements on various natural snow covers at 5.3 and 35 GHz (C and Ka band, respectively). Most of the measurements were made at a test site above Davos in the Swiss Alps at 2540 m above sea level (asl), but studies were conducted also at other test sites in Switzerland and Austria at heights between 500 and 2200 m asl. The backscattering coefficient &ggr; was measured at hh, vv, hv and vh polarization, together with physical parameters of the snow cover, like snow height, stratification, temperature, density, and permittivity. On the basis of ground information and on a simple distribution of &ggr;, we identified signatures of object classes. The signatures were used in order to evaluate the capability of active microwave sensors at 5.3 and 35 GHz for the classification of snow covers. In addition, semiempirical algorithms for the retrieval of physical parameters of the snow cover, such as water equivalent, liquid water content, and thickness of the refrozen crust, were defined.¿ 1997 American Geophysical Union