The recent discovery of chlorine-based halo–organic carcinogens in water supplies has highlighted the study of ozone as a suitable replacement to chlorine. Ozone is the most powerful oxidizing agent among water disinfectants. It has been used for water purification prior to chlorine and only its high cost has hampered its wider application. The recently discovered adverse effects of chlorine have buttresssed the use of ozone in water disinfection and today, over 1,100 water treatment plants around the world use ozone for disinfection. Large scale introduction of new disinfection systems in the developed countries will require suitable modifications in numerous existing water treatment plants. However, in the developing countries, the majority of the people do not yet have access to safe water supplies and the availability of plants which assure good quality water is ranked among the top priority targets in all national development programs in these countries.