This article deals with the widespread use of open-street close-circuit television (CCTV) systems as a safety policy tool in French cities. To investigate this diffusion, we suggest tackling CCTV as a socio-technical device able to enrol allies beyond the initial circle of technology promoters, including former opponents. Through an empirical analysis of three case studies, we show that a device can spread on a site provided that the actors in charge of the device appropriate it and discover new practical uses. As appropriation practices give CCTV supporters new arguments to justify its use, CCTV is therefore more legitimized by the possible combinations of different arguments and uses than by its strictly speaking effectiveness to fight against crime.