The increasing use of ozone in the treatment of municipal wastewater effuents has been stimulated by the need to achieve higher effluent quality and greater compliance with physicochemical and microbiological quality standards before discharge. These standards are applied when the effluent may pose a risk to the public through direct contact and where the effluent is used for agricultural purposes or water reclamation. Although various alternative technologies exist for upgrading wastewater effluents, ozone treatment may be the most appropriate approach in particular cases. This review summarizes the current status of the use of ozone for treating municipal effluents with respect to disinfection efficiency, its effect on the treatability of the effluent and on aggregate effluent parameters, the potential for the formation of ozonation byproducts, and its effect on the toxicity and mutagenicity of the effluent. The importance of treatment conditions (e.g., contact time) is also reviewed.