The use of ozone for the disinfection of sewage has been escalating since the early 1970s, primarily in the United States. Today there are 32 operational wastewater treatment plants in the USA incorporating ozonation, although only 19 ozonation systems are actually operating as of late 1981. Another 11 plants are under construction and another 9 are in design. Host of these plants utilize ozone for disinfection. Available full-scale plant operational data, data from pilot plant testing, data on efficiencies of disinfection and parameters which affect disinfection of bacteria and viruses are reviewed, as are the more salient engineering aspects of ozonation systems as utilized in treating wastewaters.