This paper summarizes the potential options applying ozone for the improvement of effluents from wastewater treatment plants. A specific focus will be on the technical aspects of how to integrate ozone technique in existing or new conventional treatment plants. Additionally some large scale projects will be illustrated, where this technique is now in operation. The following aspects are of interest and are discussed: necessary ozone dose range (depends on application, water matrix and contaminants); required components of ozone systems (ozone generator, gas supply, reaction system, off-gas handling); process control, online measurement; potential parameters to be used for process control; design guidelines for the integration of ozone systems; and cost calculations.