Increasing population, industrialization, and the desire to improve the quality of life among the world’s poorest are all contributing to the increasing demand for water of adequate quality. The competing demands for water range from ecological services, food and feed production, power generation, and shipping, to domestic and industrial needs. While the earth is indeed a “blue planet,” fresh water constitutes less than 2.5% of the available water (Postel et al., Science 271:785–788, 1996). Two-thirds of this water is tied up in glaciers and permanent snow cover leaving barely 1% available to supply the growing demand to sustain and enrich human life. Therefore, measures such as sustainable watershed development and protection, rainwater harvesting, and responsible use of groundwater sources are needed to make access to clean, affordable water a reality. While these measures will help, they will not suffice to prevent water from becoming a critical bottleneck in the development of large parts of the world that are facing population growth, dwindling fresh water supplies, and increasing pollution of existing water supplies. They must be supplemented by actions to identify, develop, and upgrade alternative sources of water to meet the anticipated gaps between the long-term supply and demand. Seawater is the most abundant water resource on the planet covering 71% of the Earth’s surface, mostly in oceans and other large water bodies. Water extraction from this salt solution (desalination) would provide a reliable source of fresh water for the foreseeable future. This chapter examines membrane-based desalination technologies with an emphasis on seawater reverse osmosis (RO). Environmental impacts associated with membrane-based desalination and efforts to mitigate them are discussed. Furthermore, the challenges of harnessing this technology to serve the needs of the poorest sections of society and the potential pathways to overcome them are explored.