The extent of the snow cover, the stored amount of water and the state of the snow metamorphism, in particular the date of the snow-melt and the intensity of the run-off, are of prime importance to hydrology and water management in large regions as well as to weather and climate. Regular observations by satellite-borne sensors can substantially improve the information on the status and the dynamic behaviour of these hydrologic parameters on a global scale. Microwave radiometers or scatterometers as all-weather and day and night sensors are excellent tools for reliable observations of cloud-covered regions of the world. The interaction of microwaves with snow strongly depends on snow wetness and size and structure of snow grains. Multifrequency observations can be used to classify snow conditions, to estimate the water equivalent or dry snow and to determine the start of the melting period. Results of several years of ground-based microwave observations along with classical hydrologic measurements are discussed and experience with satellite-borne microwave radiometers for the determination of the extent and state of the northern hemispheric snow cover is reported. Conclusions are drawn to define optimum sensor specifications for satellite-borne microwave remote sensing of the snow cover.