Uncertainties in assessing the effects of global-scale perturbations on the climate system arise primarily from an inadequate understanding of the hydrological cycle: on land, in oceans, and in the atmosphere and biosphere. Because of this uncertainty, almost all science-based initiatives have expressed the need for continued advances in global observations and modeling of the Earth system. It is in this spirit that we advocate establishing a hydrologic remote sensing observatory (RSO) to advance sensing technologies and their use in scientific inquiry into hydrologic processes. There are two fundamental reasons why establishing such a RSO is timely. The first is operational: Developing assimilation techniques to estimate unobserved fluxes and uncertainties in hydrologic forecasts has sufficiently matured to take advantage of computing facilities and detailed hydrologic observations shaped by the RSO. The second is scientific: This RSO will permit us to refine knowledge from physical and hydrologic models that can then be converted to local and global strategies for water resources management and ecosystem health evaluation. The authors outline the conceptual design, scope, and functionality of a RSO and present four examples to illustrate how the hydrologic community can take advantage of such facility.