Water scarcity is among the most fundamental, long-term challenges in the world. To an ever increasing degree, sustainable water supply depends on the utilization of water of impaired quality. This is particularly true in developing nations and in water-stressed areas such as the United States Southwest. The most plentiful impaired water resources are brackish ground water and seawater. In both, salt is the primary contaminant of concern. Reverse osmosis (RO) is the most widely utilized, membrane-based method for separating salt from water. RO treatment costs have become competitive with thermal desalination methods, even in seawater applications. However, both conventional thermal distillation and RO are energy intensive processes, exhibit economies of scale that discourage decentralized or rural implementation, require enhanced expertise for operation and maintenance, and are susceptible to scaling and fouling unless extensive feed pretreatment is employed. Membrane distillation (MD) processes, driven by low temperature generated, vapor pressure gradients, can potentially overcome many of the drawbacks associated with conventional thermal distillation and RO desalination. This presentation describes the development and testing of a solar-driven, MD process. A prototype of the process, using only off-the-shelf components, has been successfully deployed in the field. The use of solar energy for water purification is favored in areas that receive high solar insolation and do not have well-developed energy distribution grids. MD can operate using low-grade, sub-boiling temperature heat sources. When it is driven by solar energy it does not require highly concentrating collection devices, non-aqueous working fluids, complex temperature control systems, nor extensive operational expertise.