This paper reviewed factors influencing the effectiveness of ozone as a wastewater effluent disinfectant. In a brief selected review of the literature, it was shown that demand of the effluent plays an important and sometimes not so easily predicted role in defining the dose requirement to achieve a given fecal coliform criterion. Important factors include BOD, COD, and nitrite, while less important are suspended solids. The question of reliability was addressed in this paper by pointing out that control of ozone disinfection can easily be accomplished by monitoring just one parameter, the exhaust gas ozone concentration. Finally, if stricter standards are required to be enforced, combining ozone with ultraviolet light in sequence is economically more effective than either UV light or ozone alone for treatment plants greater than 38,000 cu m/d (10.0 mgd).