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Gray water (GW) has attracted global attention as an alternative water source over the last few decades. This chapter discusses the quantity and quality characteristics of GW in developed and developing countries. A number of systems varying from low-cost devices to complex treatment systems have been used/studied for GW treatment. Different combinations of chemical, physical, and biological processes such as coagulation, sedimentation, filtration, biodegradation, and disinfection have been used for GW treatment. In order to reduce cost, treatment of GW by natural systems is gaining importance in both developed and developing countries. Several factors such as type of GW, scale of treatment, land availability, reuse applications, posttreatment requirements, capital and maintenance costs, and reliability and replicability should be considered while selecting an appropriate GW treatment technology. The chapter also analyzes the health risk associated with the use of treated GW.