The studies of the hydrology of land surfaces by remote sensing from satellite altitudes are more immediately useful than those from surface or low altitude platforms. The scale is increased in hydrological studies, so the physical factors which are involved become less complex. Fortunately, large-scale, low resolution data have considerable scientific and economic value especially for planning the development of the water resources of the world. The Earth Resources Technology Satellite proved particularly useful as a test-bed for experiments with fairly direct hydrological applications. The ERTS Water Resources Working Group was in general agreement that the satellite's MSS observations provided much more useful and readily-applicable hydrological information than had been expected prior to its launch. We will summarize some of the ERTS findings and Landsat inspired suggestions for the future. Weather satellites (Essa, Nimbus and Noaa) have provided interesting data with a hydrological content in thermal infrared wavebands and the microwave region using passive sensor systems. For example, drainage networks have been traced from high resolution infrared data in atmospheric window wavebands under cloud-free conditions at night, when water and the damp soils flanking water courses maintain a higher radiation temperature than the drier interfluves. Quantitative estimates can be placed upon the differences, but by and large the resolution has been too low for the results to be of much practical use except, perhabs in a few particularly inaccessible regions. The new Nimbus Microwave Spectrometer (NEMS) gives more hope for the future. Reflectance patterns may be mapped by such means for soil muisture, snow cover, and ice type studies.