Water scarcity and stringent environmental regulations have mandated treatment of polluted, saline and marginal waters for use and reuse within various sectors: industrial, agricultural and municipal. Availability and production of water is directly linked to the nature, availability, and amount of energy required for its treatment. Various technologies have emerged or are emerging based on physicochemical and biological fundamentals. This book discusses these issues and some of the advancements and challenges in securing this precious resource for human, industrial and agricultural consumption.