Although copper-silver ionization (CSI) has been used for 30 years to inactivate Legionella bacteria and other opportunistic pathogens in water, the literature is a mix of both successes and failures. This paper reviews the technology and case studies to help improve the success of CSI installations. Important is a properly designed system capable of consistent delivery of copper/silver ions at their target levels. However, even the most advanced system will fail if not properly operated and maintained. Water chemistry can impact the performance of CSI systems and attention should be on conductivity, temperature, oxygen, flow, pH, chloride, sulfate, alkalinity, hardness, phosphate and dissolved organic carbon levels. Several case studies are provided to demonstrate the effectiveness of CSI treatment even at high pH levels. The report concludes that the use of CSI to control Legionella and other opportunistic pathogens is highly effective when the units are properly designed, maintained, and operated.