Combined sewer overflows are a known source of aesthetic pollution in urban watercourses, and screens have traditionally been used as a means of preventing such pollution. The authors have established the effectiveness of combined-sewer overflow bar screens in preventing the visual pollution of rivers and have determined the factors which influence screen efficiency so that guidance can be given on improving the performance of future installations. A field study has been undertaken to monitor existing bar screens with different bar spacings. The hydraulic performance of the overflow chambers has been established, and the sources and type of visual pollutants have been identified. Analysis of the field data has established that screen performance (for the main polluting solids) depends on (a) screen bar spacing, (b) mean rate of flow through the screens, and (c) frequency of raking. Overall, mechanically raked bar screens are likely to achieve retention efficiencies of less than 50%.