This article examines the performance and effectiveness of ultraviolet (UV) irradiation for disinfection of combined sewer overflow (CSO). Due to the negative impact of conventional water disinfectants on aquatic life, new agents (e.g., UV light) are being investigated for CSO. This low-quality water with high flow rates, volumes, and suspended solids content requires the use of high-rate ... [Show full abstract] techniques for its disinfection. Although many pilot-scale studies have investigated UV irradiation as an alternative technology, to date no full-scale CSO treatment facilities in the United States are using UV light. A survey of the major pilot-scale studies investigating UV light as a CSO disinfectant suggests that UV light irradiation, correctly applied, is an effective alternative to chlorination for CSO. The success of disinfecting with UV light seems to be strongly dependent on water quality. Thus, pretreatment of CSO prior to disinfection is a major prerequisite to ensure UV light effectiveness.