Publisher Summary This chapter discusses the various methods used for water disinfection. Some form of disinfection is necessary to ensure the elimination of potentially harmful micro-organisms from potable waters. Chlorine and its compounds are widely used for the disinfection of water. Ammonia may be added to waters lacking in ammonia to allow the formation of chloramines that tend to cause less trouble with tastes and odors than done by free residuals. Ozone is an efficient disinfectant and useful in bleaching color and removing tastes and odors. Like oxygen, it is only slightly soluble in water and because of its unstable form it leaves no residual. Ozone must be manufactured on site by passing dry air through a high-voltage high-frequency electrical discharge. Various forms of radiation can be effective disinfecting agents and UV radiation has been used for the treatment of small water supplies for many years. The disinfecting action of UV at a wavelength of around 254 nm is strong provided that the organisms are actually exposed to the radiation.