This study examines the effectiveness of a closed circuit television (CCTV) system installed in Burnley, Lancashire in northwest England. It considers both the outcomes and mechanisms through which they were brought about. Three areas are identified: "focal" beats, within which the CCTV cameras were installed; "displacement" beats, which were continuous to the focal beats; and "other" beats, comprising the remainder in the police division. With regard to both overall recorded crime and separate types of offences, the research finds significant decreases in the focal area, no spatial displacement, and some diffusion of benefit to the displacement area. There was some dilution of impact over time. There was no evidence that the proportional effect of CCTV changes by time of day, according to periods when surveillance with cameras would be more or less difficult. Crime fell more steeply as the first cameras were installed, with diminishing increases in effect as more were put in place. These patterns suggest that the impact of cameras is not simply a result of surveillance effects per se. Other preventive mechanisms were also triggered.