Membrane distillation (MD) is an emerging technology for desalination. Membrane distillation differs from other membrane technologies in that the driving force for desalination is the difference in vapour pressure of water across the membrane, rather than total pressure. The membranes for MD are hydrophobic, which allows water vapour (but not liquid water) to pass. The vapour pressure gradient is created by heating the source water, thereby elevating its vapour pressure. The major energy requirement is for low-grade thermal energy. It is expected that the total costs for drinking water with membrane distillation will be lower than $0.50/m3, even as low as $0.26/m3, depending on the source of the thermal energy required for the evaporation of water through the membrane.